Iron on transfers for dark shirts?

[coastgirl]

So I had a great idea to do some "house" tshirts for the Wizarding World at Universal. The house colours of the two designs I want are red and gold, and blue and bronze. I found some cheap, nice tshirts in great dark colours. A maroon red and a royal blue. I tried the transfers a couple of different ways, but it just comes out uniformly dark, so that you can only see it if you know it's there, and with no discernable colour. Is there anything I can do about it? I thought I had "dark transfers", but apparently they don't work like I'd hoped they would.

On the other hand, we did freezer paper stencils on the front with gold/bronze animal silhouettes, and those look fabulous on the dark shirts.