My right leg is slightly darker than my left leg when i'm sitting. i don't go tanning. should i be concerned?

The: Whole leg coloring discrepancy would be rare unless your work require you to drive around on passenger seat and you where wearing shorts. Should see a dermatologist for a closer look.

Possibly vascular.: If this difference in appearance of the legs is only associated with a particular body position, this suggests a possible vascular cause (blood vessel; either venous or arterial). It would be prudent to schedule a visit with a physician for a careful history and physical examination. It may be that as a result of this evaluation, further studies may be needed to completely understand this.