You can't just make it, you have to breed it in from a breed that already has it, and it can be a bit difficult and take several year to breed it in and then get the correct type in the breed you want. But basicly you find a breed that is already lavender and closest in type to the breed you want to create the lavender color in, and breed the lavender bird to a black bird of the desired breed and that will give all black ( split lavender, carrying only one copy of the lav gene.) chicks, the cross those chicks together and about 25% of them should be lavender, then cross the best lavs from that cross to the pure black breed you are wanting and that will start you over again and keep doing it until you get the correct type. BTW There are already lavender cochins so you would be much better off just buying them.