Can I replace LEDs in store bought sets with my own LEDs?

[Blackbeard]

Just wondering if there's a trick to replacing LEDs in store-bought light strings with my own LEDs. I've got some cheapo fixtures that have 5 or 6 white LEDs in them on a 120V string (One of those tree top stars from Walmart). I've got a few RGB LEDs that I'd like to experiment with.

Can anyone suggest a way of determining if these will work and what means I need to figure out current, resistance if needed, etc. or this just a bad idea?

[SteveMaris]

I use this..