I've noticed that my veins stand up when my hands are really hurting, but I don't see a difference in the color. Maybe it just looks darker to you because it's pushed up closer to the surface of the skin. I think they do that because of the muscle tension when it's in pain... just another odd symptom to me. My veins are already on the surface so that may be why I don't see a difference. Makes getting blood drawn very easy... woo hoo... ha ha
Hope this helps you figure it out.