There are two great posts on Gardening Gone Wild, Gardening in Black and White by Nancy Ondra and More on Garden Tones - and Bones in Black and White by Steve Silk. Both talk about the role of tone, or where a plant combination fits on a greyscale, as another factor to consider when combining plants. Nancy has numerous color/greyscale versions of plant combinations in her post that illustrate how things other than mere color play a role.
Of course I immediately raced to Photoshop so I could do some comparisions! Here is my front yard, where I think I have actually gotten too carried away with foliage color and contrast, but the contrast looks good in greyscale.
Here is a design by my friend Roxy who I mentioned previously. I love the contrast of this combination in both color and greyscale.
This shows the important role the yellow Santolina plays in waking up the combination. Not one of my favorite plants but I may re-evaluate.
I encourage you to check out the original posts on GGW as their explanation is much more detailed - I just wanted to mess around with some photos. If you decide to try this on your own garden, leave a link or email me a photo to post - I'd love to see your results.