Greens seem to be a difficult color for many painters.  In this tutorial I offer one of my standard methods for painting greens that are harmoniously related throughout a landscape.  This uses the method of using yellows and blues to make a green, but it could just as well be applied to using a pre-mixed green and tweaking it with other colors.

Preparation:  If you are doing more than the first step (base tone) and you want to work wet-in-wet, you should have all of your mixes (base, middle, and dark) prepared at the get-go.

Base tone: First, start with a base color/tone.  If are doing an area that needs relatively one tone, this is the only step you will need.  Here's what I do:
    I make a mix of whatever yellow and blue I need to get the color and brightness that I want.  I make sure it is a variegated (not one homogenous color) mix.
      As I begin to paint, I constantly tweak the mix by adding the two initial colors: yellow, blue, yellow, blue, yellow, blue... to keep the colors varied.  I change the amounts of each color about every two or three brush charges.
    The tricky part of this is that once you've begun, and you go to add yellow or blue to your mix to vary it, you are changing the consistency of the mix as well as the color.  So, if keeping the consistency is important (which it almost always is), you usually need to add water to the mix as you add the new paint. 
    Here's the bad news:  This is one of those times that being very confident with basic techniques is essential, and honestly, it is one of those things that just can't be done without a lot of practice.  Getting your mix consistency must be second nature, and you must be able to dip that brush into the paint, wipe the right amount onto the palette without changing the color too much, dip the brush into the water, perhaps dab it on a tissue, and then add just enough water to the mix to make the consistency right again.   And it all must be done FAST in order to keep your wash wet and juicy and to not have it drying on you.  The action must be just dob-wipe-dip-wipe in a matter of a couple of seconds.

Middle tone: If you need a deeper tone, say for defining shadows in a tree canopy, move on to this step:
    I make a separate mix of darker green usually made from Hooker's Green (any Phtalo based green will do) with touches of yellow or blue to approximate the greens used in the base tone. I use this mix to drop in at the "right moment" (a whole other topic) as the base tone wash is drying.  I keep this darker mixed varied too, and adjust it by adding one of the two colors about every two or three brush charges.

Darkest Tone: If you need deep darks, such as at the base of a heavy tree cluster, go on to this step:
    I darken my base mixture and/or my middle tone mixture even more with a black neutral tint and drop these in at the "right moment" for super darks My method for this is where I diverge from many painters.  Here's what I do:
    I always have made my own neutral tint instead of using Payne's Gray or other carbon based blacks.  I make this by mixing Hooker's Green (Phtalo) or some other very strong green with a strong compliment like Quin Rose, Dioxazine Violet, etc..  This gives a very dark, almost black green, and viola, your darks are adding even more variations of green.  And to boot, it doesn't dry so much lighter, like carbon blacks.  And, unlike carbon blacks, it is semi-transparent which allows some of the others greens underneath to show through.
    Recently, I have discovered that M. Graham's neutral tint is not made with carbon black, but with Phtalo green and Quin Violet (the same exact method I was using). I have never tried Phtalo Blue and Burnt Sienna for this, but I assume it would work well to, as it makes a very dark green (although it is usually a cool green so this might not blend with the other base and middle tone greens used earlier).  Winsor and Newton now offers a Perylene Green that is an almost black green right out of the tube.  I have used this, tweaking it a little to fit my base and middle tone greens, and it works brilliantly as well.

Here is a sample painting with my tonal sections labeled: