Another easy way to adjust color variation is just throwing a scalar parameter for the diffuse, then create a few instances and tweak the param number and select a bunch of foliage in the level and apply the instances on a per object basis.
No, you can't. a 3rd party application like Beast can, and does. Hold on a sec, I think if it's a 1-bit alpha it will, blended, it won't. If you want to add a color variance to the plants, use vert colors and some type of noise added to the spec/diffuse channel. That combined with lighting in the level should break things…