Hi, sassyk!
Sure, you can mix colors. When I was just starting to gather supplies, I did exactly what you're talking about - I got red, blue, and yellow and figured I could make other colors out of those. If you're confident in your blend, you can mix your colors right into your base, but if you're unsure how something will turn out, you can always mix a bit of color into a small cup before adding it to your base. That way, if it doesn't turn out the way you'd like, all you've wasted is a few drops of color.
Mixing colors worked out well, but eventually I wanted some colors I couldn't make easily, like black and brown. And it got to the point where it was just easier to have green or purple on the ready rather than mixing colors. The bright pigments and sparkly micas called to me, too. Now I've got all kinds of colorants. I've mixed liquid colorants, powdered pigments, and micas to get some neat effects.
BB's got some nice sample packs of colors:
http://www.brambleberry.com/Sample-Packs-C41.aspx. The packs last a while, and you can see what you like and don't like for a good price.
Have fun!