While working with color, we tend to get wrapped up in hue and saturation, what colors we use and how vivid they appear. But ever notice that something you've been pouring color into just isn't working out? Try slapping a desaturation layer on top of it to see how the contrast is holding up: often you'll find the problem is that without color, there's not enough contrast for the color forms to keep from blending into one another.
I had such a problem on one scene in my ongoing project, Alice in Videoland (http://rachelnabors.com/alice-in-videoland/book). I had a scene where the Hipster Rabbit pops down his hole. I work in three shades of grey when I rough out a storyboard (rachelnabors.com/alice-in-videoland/storyboard), and I'd put a dark Alice in the foreground, a field of high-toned grass between her and the rabbit, and a medium-toned background.
The problem was that the tree needed to stand out from the foliage behind it, and I just wasn't getting anywhere working with color. My solution: Using colors from the scene's color guide, I worked underneath a desaturation layer to get the below. I ended up darkening the foliage just around the tree and adding a "halo" to Alice to bump up their contrast.