Actually, I think lavenders and yellows look nice together. Purple and yellow are opposites on the color wheel, and opposites are often paired together by designers.
Try this site to see what color groupings you like:
http://colorschemedesigner.com...
To use the wheel, start by clicking on any color inside the wheel, then click on one of the little wheels above the big wheel to see monochromatic complements (shades of the same base color), or complementary contrasts, or triads, etc. It's a lot of fun to play around. Of course, not all colors are available in roses, but you could come up with non-rose companion plants of nearly any color on the wheel.
