Howdy Guys and Girls :)
Today Andy answered the inconsistencies between the Knight colours in Titandeath and already published older artworks. Basically, the lists in the supplement represent said Knight Houses at that time of the Horus Heresy, and some of them changed their colours. Prime examples for this would be House Rajha who went from ghost grey to orange or House Devine and their sudden love for purple. At the end, this just opens up more possibilities so you shouldn't mind it too much.