I voted yellow (here 6 years later), because I tend to mix yellow or orange into every other color, but looking more closely at the art i haven't posted, i've done most of my underpainting in pink, because it layers well for skin tones
I use a lot of blues and skin tones (flesh color, orange, red). Recently I've been finding that mixing red or purple with my colors produces some really nice hues, so they're becoming some of the most used colors in my palette.
So far, since I've been obsessed with skin tones and drawing chibi characters, it's definitely been pale shades of warm yellows, browns and reds, all basically rather earthy colors. :3 I voted "warm yellow" since that's usually my base skin color.
As far as pigment goes, I use tons of reds...mostly alizarin crimson. However, I paint primarily with coffee and tea, so naturally there's lots of browns. I really love warm palettes. I try not to discriminate, though.