Here is a very simple technique for rendering bushes, and can be applies to trees, or other plants. I use this technique more for bushes that are in the background, that you don't want to stand out or don't need much detail.
Coloring Simple Background Bushes
1. Start by picking 3 colors. One nice thing about greens is that on a plant you don't need to be as careful about following the Natural blending family, as long as you have a light, middle and dark that aren't too different.
Leaves in sunlight tend to look more yellow and bright so I usually pick a light YG in the 00's or 10's with a last digit of 0 - 3 for my highlight, and shadows tend to have more gray, so I go with a G or YG in the 60's-90's with a last digit of 7, 8 or 9
2. Think about the plants around your bush. If your bushes are exactly the same colors as your grass they may look a little strange, so I try to vary the two (I talked about coloring grass earlier). Have some bushes lighter and some bushes darker so people can tell they're not the same kind of plant.
3. Choose a highlight. Usually on top or slightly off to one side.
4. Begin Coloring
There are few main ways I do bushes, but the ways I'll cover today are Smooth, Scribbling, or Stippling. The main thing to keep in mind is that bushes are irregular. Individual leaves pick up the light from different directions, but if you try to carefully draw every single leaf on a bush you'll go insane. So my methods are for simple coloring that gives the illusion of complexity without the hard work.
Color with your lightest color first. For smooth blends, layer your next color while the base is still wet, add your next darker color, color back over the edges with your light color, then add your darkest color, again going over the edges with your middle and light colors until the edges disappear. For crisp edges between color layers let each color dry completely and don't go back over the edges with a lighter color each time. Too bad this doesn't look very believable- it's just a smooth lump of green, not really a bush.
Scribble the base color on, since the uneven coloring helps accent the roughness of leaves. Then layer on your midtone, again coloring unevenly, and last add your dark. How much or how little of each color is a matter of personal taste. I like this method because I can leave some spots of white- areas that act as natural highlights, and it's much quicker than smooth coloring.
Dot on your lightest color, leaving lots of white, Add dots of your darker color, and finally finish up with your darkest color. If you are using a Ciao or Sketch marker this is the easiest technique, and you have two options. You can have either round dots by mushing your tip straight down, or you can have individual leaves by using the side of the brush marker.
Note: scribbled and stippled bushes will draw your eyes more than smooth bushes. Why? The crisper lines and bright white spots attract our attention more than a smoothly colored bush. So if you don't want the bushes to attract as much attention, leave less white areas and make sure the base color is more muted. Then the parts of your picture you care more about will stand out more. Remember also that our eyes are drawn to things with more contrast. If you don't want people to focus on your bushes then color them with less contrast.
Also, if you color something in front of these bushes that has super smooth, beautiful blends and then you have scribbled or stippled bushes in the background it will look strange. In this case you've lost your continuity. It's like the bushes were an afterthought.
Here I want the feel of individual leaves, but I want them to not stand out as much, so my first two color layers are smooth, then I dotted on the darkest green. Now you get the feel of individual leaves, but it has less sharp contrast than stippled by itself. This technique was very quick, it goes with more foreground elements than scribbled or stippled only, and is not as fake looking as smooth bushes.