There's no real rule of thumb except "check out how light works in real life". That's really the guiding force in "real" pixel art - real grass doesn't actually go from dark green to light green. Yellow from the sun, blue from the sky, the green chlorophyll, brown dirt, tan/white of dead chlorophyll cells, all these things contribute to a really wide range of color values, even in something as "simple" as grass. Shadows are never really black - blue is a very pervasive color in our world, because of that whole "sky" thing. So even when there is very little light (like under a plant) outdoors, there is still a lot of blue! Also, for illustration purposes, blue is a good shadow tone anyways because it receeds (this is not a real "rule" per sec, just a useful optical effect).
These are all the things that you should process when you're sliding your colors around!
