...unless Rob means the billboard grasses. It helps to color the lower 3rd of the billboard in the same hue as the texture below. I like to make the tips a bit brighter, to get a bit of a sheen effect going, making the grass look a bit fluffy. Also, with billboards, you will never get a decent grass coverage without planting 3 or 4 differently shaped billboard of the same type of grass into mixed zones, so that the single units and their many repetitions are not recognizable as such any more.
Trick: In Unity, just doubleclick on the billboard or the texture if you want to make some color corrections (those inside Unity are not worth it) and you asset file will immediately open in your graphics software of choice. Edit and save it, and Unity will auto-update all instances inside your scene. The original file will not be altered, btw, just the copy of it inside your asset folder.
But, as always, shimonko is essentially right: There is only so much you can do without CF - and you have done most of it very well. Very linksy mood you've conjured up, me likey! - Detailed adjustments are best made with CF in place, with their meshes, textures and shaders added into the mix. Because once you change only one thing even one tiny bit, everything else will interact with it differently.