Black is pretty basic and is the cleanest color to work with to produce a good consistent plumage color on lavender. Buff has a lot of extra genes involved. The first cross would produce many different colored chicks. Using blacks produces consistent color (or should, as long as there is no red leakage). Selections should be based on multiple factors - a few of which are body type and color. Matching up birds is typically based on individual perception and goals. I have bred split to split, lav roo to black hens, Black roo to lav hens, etc. We are now taking our best lavender roo back to some great looking black hens to continue working on our line. Matching up birds to get the right result is not just a matter of putting a couple birds in a breeding pen, but finding the right balance in the father and mothers to give the best results.
Good luck with your hatch.