When you're in 'edit signature' click on the A with the black line underneath it in the tool bar at the top. It will show you a range of colours to choose from.
Be aware though that every change you make from standard font, size and colour uses a lot of characters because they all have complicated codes with them (which you can't see). As your signature is limited to a certain number of characters you may not be able to get in everything you want. I had to change everything back to bog standard and lump some weeks together to fit things in.
Hope this helps x