Changing the color of part of a design

Hi!

I have a design and I need to change part of it a different color.  It is one design, so in order to change the color do I have to break it apart?  Because I'm using the design for a vinyl cutter would I then need bring things to the front and do the front minus back on it all again?  I hope this makes sense, when you break a design apart it messes all the parts of it up esp for cutting.

I have never been able to figure out how to change just part of a design without breaking it apart.

Thanks,

Tracee