Not really. You can in the tanning process of course but it is much harder afterward.
Think about having a brown felt pen line and then going over it with a black pen. You could get the line to go black. But if you have a black felt pen line no amount of going over it in any lighter colour is going to make a dent. You can go from light (relatively speaking) to dark; but not the other way.
What size kit did you use ? I need to do a flash noseband & a running martingale. The small kit is £ 35 which seems ok as I am not sure I woudl get both of the above in English leather for that ?