I have a suggestion that might be interesting to try the next time 'round. You start in back and white, then paint the colour pretty much alla prima directly on top of there right? Why not, at the step right before you add colour, you change the colour of the black and white image to some whack colour, maybe the opposite of the colour you're going to paint on top, then pretty much do your thing from there on out. I find it helps for colours to pop out, and really have a great variety and vibrancy to them. I picked the tip up from reading books on oil painting, and watching some of Shaddy Safadi's YouTube videos.
Thanks for putting up the process by the way, it's always interesting to see how other people work!