QUESTION: Do you have to match the color of the thread to the fabric?
ANSWER: No, not necessarily. (See below)
In my opinion, colored thread is best used when you want to accent a seam, be artistic or camouflage the stitching. I don't think it's always necessary to match the thread to your fabric. In fact, it's usually more practical to have a good supply of neutral-colored threads on hand because they'll get you through most projects--off-white, black, tan, and gray.
My favorite color of thread is gray; it's subtle and pretty all at the same time and seems to be a chameleon color as well. Don't get me wrong, I love color! But when it comes to sewing with colorful thread, I reserve it's use for topstitching or any visible stitching. I especially like colored thread if I want to add an accent to my work but most often I use neutral colors.
So, if you're looking for practicality, go with gray!