Assuming the patch is smooth and taunt applying second color trim with the same covering material (Monocoat is my preferred product) is much easier and works quite well if you vent the base covering.

On occassion for this I use a dedicated tool called a woodpecker but you can also do the same just by using a dresssmakers pin (very thin) and makiing many tiny pin holes on the entire area where the second trim piece is to be applied. It makes no differance whether or not it is over balsa or open bays and it works equally well over both.

John