Flower petals contain lots of water they have to be dried or dehydrated in one of those food dehydrators. The thick phone book trick also works, it may that a little longer tough...!
As for the colour being affected by the resin heat/reaction, there are a few products that are used to seal the surfaces of various materials, from stamps to flowers, these sealer's are normally very effective on protecting the colours.
Bigger or smaller petals/pieces, everything in moderation, bigger may require some extra degree of difficulty but, all is possible, the main goal is to void the air bubbles to get trapped in the resin as it sets, for that, a pressure pot does a good job...!
Good luck,
Cheers
George