Hi Jeff, there is this chart for predicting your seedling's color. http://www.love-of-roses.com/predict-rose-seedling-color.html

The best book about breeding for "funny colors" is Rose Growing Complete, by Edward LeGrice. You want this book! Not the 1965 hard cover, but the updated 1972 soft back edition. He has a short chapter providing the recipe for breeding for gray and brown roses. He said you need an admixture of red, yellow, pink and a bicolor red/yellow to produce the lavender. Homogenizing the genes, say lavender x lavender, leads to heavily greyed whites, almost silver. Throwing in a ringer to partially break down the homogenization, gives browns (russets). If the "ringer" was of R. Californica breeding, it provides stability to the browns and in the cases of his breeding (Vesper, Jocelyn, Victoriana) it leads to cluster flowering with characteristic short, bunched stems. He further wrote that changing the red/yellow bicolor to one of red with a white reverse, led to vinous purples such as his Great News.

The only case I know of where an odd color resulted from a cross which appears to have behaved like mixing paint is Julia's Rose. Lavender (Blue Moon) X golden yellow (Dr. A. J. Verhage) created "copper and parchment", almost like a brown paper shopping bag.

You're probably aware of instances such as wall flowers, pansies, violas, primulas and mums where purple varieties frequently sport to russet tones, but similar things have occurred in roses. Winter Magic, one of the heavily greyed whites which appears pale, silvery lavender, sported to soft coffee and cream, called Cafe Ole. I'm sure there have been others, but this example comes to mind. There is a strong correlation between purple and brown in flower colors. I'm sure you and others can come up with many other examples.

Ralph Moore stated frequently that to 'stabilize' purple roses, cross them with strong yellows, those with little to no fade. To keep the purple (or lavender) "clean" or unstained, you would need a yellow which doesn't stain red. Potential colors are indicated by all the plant parts. A rose whose new growth is strongly red, even though the flowers don't express red, can produce a reddish seedling with the right parent. If you have a lavender which fades too much for you, crossing it with a yellow which doesn't express any red tints in the plant or flower and which doesn't fade much, can help to intensify and stabilize the lavender. Crossing it with a stained yellow leads to lavenders which blush or burn red, such as Angel Face, Paradise, Barbra Streisand, etc.