Currently (as far as I can tell) there is no way to see your custom team uniform until you play a game after you've changed your colors. However, once the games start (even preseason games) you can no longer change your colors until the next season.
The result is that in both my Custom leagues, I am currently playing with uniforms I don't like. It would be nice to have a way to see your custom colors on your uniform without having to wait for a game.
I know this is a relatively minor feature, but since I have to pay extra credits to use it, it would be nice to see this changed.