I am making a Reveal Cake for a friend and ordered pink and blue melts.  After making the modelling chocolate (for the first time) the colors are really more bright than I would like.  I want lighter "baby" tones.  Can I work melted white chocolate into the already prepared modelling mixture to tone down the colors?

You'd be better off to make white modelling chocolate and mix it with the candy clay colors you already have.

Perfect, that is exactly what I thought!

It came out kind of greasy when I kneaded the white and color together.  Is this the normal texture?  I used one pound chocolate melts with 1/2 cup corn syrup and kneaded them both colors together with lots of powdered sugar.......

For 1lb of chocolate I use no more than 1/3 c. of corn syrup and no powdered sugar. I've never heard of adding powdered sugar to it.