It can be very rewarding to mix green, but also one where you need to take notes about what you do so you can remember how to mix it next time! The green you get depends on which yellow(s) you mixed with which blue(s). To get a lighter tone green, try adding yellow, not white. To get a darker tone green, try adding blue, not black.
Pablo Picasso is quoted as saying: "They'll sell you thousands of greens. Veronese green and emerald green and cadmium green and any sort of green you like; but that particular green, never."
If you want to lighten a red, you'll most likely automatically reach for white paint and end up with a range of pinks. Try mixing red with a light yellow instead of only white.
Yellow is one of the hardest colors to visualize in a tonal range, as even a 'dark' yellow such as cadmium yellow deep seems 'light' when placed next to many other colors. But while you won't get the same range of tone as with, say, Prussian blue, you do still get a range of tones with any yellow.
Read More: Color Theory FAQ: How Do I Mix Greens?