Colour project advice?

A while back I started a new colour project with clydesdales. My focus is breeding for leg and face markings on a solid base, and I chose my foundations based on how many white markings they had. Most have three or more.

Since this is a colour project I'm expecting to need to breed them quite a bit more than a stat project in order to get at least one or two suitable foals, with quite a few rejects left over. My question is, when should I start breeding them? They're currently 14, and I'm not sure how reliably white markings pass along to offspring.

I am working on a similar project with my QH's and have had varied success in getting leg/face markings passing to offspring. I breed my horses starting around 17 or 18. It is a tricky gamble but I wish you luck!

This colt is one of my successful outcomes: //nyx// Impressive* 
(his dam had 0 leg markings with a blaze on her face, his sire had no face marking but 4 white legs under his overo pattern)