hiya pal, the bulbs are located in small plug type things that just twist out of the rear of the guage, have a look, they will light up red anyways since its from an audi, if you want another colour, it not as simple as changing the bulb im afraid, there is a red filter inside the guage, and the only way to change this is to open up the guage, and replace the filter with a home made one...
but since you wanted red anyways, sounds liek you wont have to do this...
hope this helps...