Weather penetration should never be predicated on the color on the radar screen, alone. It's a given that on a magenta-colored return, penetrating the core into level 5-6 weather is extremely hazardous and shouldn't be done, but the black space beyond that magenta return might be much worse. In that case, the heavy precipitation in the magenta area may be attenuating what lies beyond; simply because it isn't painting yet doesn't mean there's nothing there. The point is that it's important to consider the total, or big picture, which involves situational awareness, use of tilt, gain (where appropriate), and an understanding of weather theory, as well as actual reported and forecast conditions. Frozen precipitation doesn't paint well, but can be extremely hazardous. Precipitation can come from above, or below. Rising air on the upshear, or upwind side of a cell, can be equally hazardous to the descending air in or on the downwind or downshear side of the cell. Weather won't paint in an overhang in many cases, but there may still be hazards there, such as hail. In fact, regarding color, one might see a green return along an overhang, but encounter hail significant enough to shatter radomes, windscreens, or damage or shut down engines.
When looking at a return, look at the winds aloft, both forecast and what you're seeing in real-time as provided by your instrument data (bearing in mind that what you see right now may not be what's going on when you get closer to the cell (especially if the cell lies on a frontal boundary). Mild colors and level 1-2 returns (green), or 3 (yellow) may be indicating downwind of the cell, but may in fact be quite hazardous, despite fairly benign returns.
Look at your range and altitude, and tilt. When there's weather, move that tilt around; see what is above and below. If I'm sitting at 35,000', I might put the bottom of my radar beam at 80 miles; what's seen beyond 80 miles may actually be below me, as I'm looking down through the weather. If it stays a threat inside of 80 miles, especially inside 40, deviation is warranted, but it's well to keep in mind that the the weather is dynamic and over a short time it can change considerably. As noted before, the rising column of air that rolled me, with shaker and pusher, was approximatley 12,000 feet per minute vertical rate. What was below moments ago may. now be above. A few days ago I got video of a cell ahead of us that was below, and grew well above us at 35,000, as we approached. Setting tilt is useful in cruise, but varying tilt not only when working around or approaching weather, but when in cruise, is also not only useful, but important, as is understanding what that tilt is doing and what's being displayed as a result.
Most radars have auto squelch or auto gain settings, as well as variable gain. I see a lot of guys who take their radar off the auto setting (where it should be, most of the time), and play with it, not knowing what they're doing. What your'e seeing when you'e made manual adjustments can't be compared to what you would have seen in auto, and can lead to some bad choices.
Your question is about penetrating green and yellow returns; setting aside that not all radars display or provide the same information, and older radars (monochromatic) don't feature color coding, yellow and green provide a scale of dbz returns (decibels). Where are they in relation to the cell(s)? Are they changing? How do they change as you tilt up or down? Are you in freezing conditions, or are you looking up or down into freezing conditions? Might you be looking at frozen precipitation, which gives a lesser return? If it's got velocity but is frozen, you may be looking at hail or other dangerous precip that may also have high velocity. Alternate the use of turbulence settings, which give some measure of velocity of precipitation; it may also change your perception of what you're seeing. If the yellow and green is wide or narrow, this may also give an important clue as to what it contains; a narrow band of yellow and green next to a red or magenta return might indicate a rapidly changing gradient in moisture content, and provide some clue as to intensity, but simply because that's where the moisture stops (at the edge of the return) doesn't mean that's where the weather or hazards begin or end. Going between two cells can be quite hazardous.
If you're talking about green or yellow returns on their own, not associated with other weather (such as a defined cell), then the yellow may represent the areas of avoidance, but it won't be linear looking up and down. It may move as you tilt, meaning the column of rising or descending air is also tilted, typically with the wind. Remember that wind doesn't blow a cell, but blows through a cell; it's constantly forming and dissipating, even though it looks like a cell defined by the visual appearance of the clouds. Air is blowing through it; wind is blowing through it. Air may be rising or descending on either side; typically it's rising on the upwind, or upshear side of the cell, though not necessarily, and not uncommonly, the air is descending on the downwind side...though again, not necessarily. The yellow return identifying an area with greater moisture content (and energy) may show a base in a different location than upper levels as you adjust your tilt; typically this will lean the same direction as the winds, but bear in mind that wind direction and velocity changes with altitude and other factors. Look for changes in the return with time, especially significant changes in a short period. What looks like a harmless return may not be in a few minutes.
Vertical velocity is cumulative; a parcel of air rising at 1,200 fpm hits a parcel of air rising at 500 fpm, and may shortly be doing 1,700 fpm. The addition or loss of moisture and the increase or release of energy imparts changes in vertical velocity that can cause rapid changes.
Icing should be considered, and viewed as multiple potential threats. Engine icing is different than structural airframe icing, and frozen precipitation another threat; all are considerations when looking at radar returns. Is the return benign because there's nothing there, or because it's frozen, and if it is frozen, it it frozen at higher altitudes, falling? Frozen at lower altitudes, rising? I see a lot of assumptions in the cockpit about icing and the state of precipitation; most notably, that it's too cold to be liquid. No, it's not. Look out the window at that cell at 35,000' and see if it's sharply defined puffiness, or more of a cottony, fluffy look...the latter is usually glaciated (but can still quickly ice an engine under the right circumstances); the former has liquid precip, or should be presumed to have liquid or supercooled precip; it can change state rapidly and that may be deceptive on radar, too. The worst Icing I've encountered on research flights has been around -15C, typically with high moisture contents (around two grams of moisture per cubic centimeter, though you won't be able to tell that with conventional instrumentation). I've seen it build rapidly enough I can see it move, inside fairly mild returns without an actual falling precip..the moisture is in there, and the cloud has a lot of energy, but it's got a mild return...good place for ice, under the right circumstances. Same for flight above cells, or flight in ice crystals at higher altitudes, and those circumstances produce very mild returns.
Look at your circumstance. Do you have a bleed inoperative or an engine shut down? That should temper your thinking regarding what you're seeing, and is all the more reason to be active on the tilt as you explore not just a small cross section of the space ahead of you but above and below it. Get a good picture of what you're seeing; the dimensions and returns will change as you tilt. Make small, incremental changes, give time for a sweep or two, and look closely for changes from one sweep to the next, or over several sweeps.