Can I successfully graft a 1-1/2" branch from a dark purple Lilac onto a 1-1/2" branch of a lavender Lilac? I presume the graft should occur while the sap is down and the leaves are off. Should I use a V-shaped cut, and tongue and groove the graft to the root stock, clamp securely and guy wire the scion to help minimize movement?
People do, certainly, successfully graft lilac. If you want to try it, I suggest you make several grafts, as not all the grafts will take. Also, the more research you do, the better your chance of success. Here are a couple of articles to get you started: https://www.gardeningknowhow.com/ornamental/trees/tgen/grafting-trees-what-is-tree-grafting.htm