If it's just a hue variation over a population, it's very easy; in the color tab of a population you can add fractals or image maps, or other masking (altitude, slope, etc, by distribution shader).
If you need to change the color completely over a population, say from green to orange randomly a simple addition wouldn't work (an orange fractal would just turn the greens into drab grey-brown), but there's an easy way within the object of that pop.
1. add a surface shader below the default shader and mask it by whatever mask you need, but pulled through a transform shader set to world position.
2. add a (blue node) adjust saturation node to the color input of that surface shader, and pull a line from the default shader into the blue node. Add a (blue node) constant set to zero. This turns original color into greys. If greys are too dark, just set color to 5 or so instead of default.
3. add a color adjust shader to the child of the surface shader and set different color in gamma. This will change the unmasked part into the new color.
4. beware that any translucency is lost by the surface shader, so you might need to add a Lambert shader to the child as well. Check 'use existing color' and the translucency needed.
5. might be that reflectivity is alo lost (I'm not sure), so you might need to add a reflectivity shader in the child line as well (set to non-RT).
This is one way. I hope this will work (easily) for you.
