Depends. You could do it with a gradient, that would allow you to keep the text live, but you'll have to do a bit of hoop jumping to do it. Not too much, but your question makes me think your probably not yet at the level of comfort with illustrator that hoop jumping requires.
So instead, you'll need to turn the text into outlines and then slice it into pieces and color those pieces. When you do this though, be aware that the text is no longer text, it's art and therefore can't be edited.
select the text
turn it to outlines by typing command-shift-O (control-shift-O on a PC)
choose the knife tool from the tool panel (not the slice tool, which looks like an exacto blade, but the knife tool which looks like a steak knife. It's hidden under the eraser tool)
drag the knife tool through your letterforms making sure to start and stop outside of the letters so you cut all of the way through.
using your direct select tool, select the pieces you want to color and choose the appropriate color swatch.
Hope that helps,
Sent from my iPad