Hmm, the easy answer for color portraits would be the fauvists/expressionists: Henri Matisse, Kees van Dongen, Alexej von Jawlenski, and others. But as for more realistic portraits, here are other options:
Beauford Delaney: used slightly exaggerated pallets in his portraits
Alice Neel: often colorful portraits with smooth brush strokes
Jenny Saville: Slightly distorted portraits with patchy color
Chuck Close: known best for photorealism but also made colorful works
Andy Warhol: didn't do much painting because he focused on prints, but his color filters may be inspiring
Martial Rayse: "gaudy" prints of classic paintings with bright color filters
Paul Gauguin: started a whole mini movement with his color treatment!
Pablo Picasso: known best for cubism, but also made realistic portraits with exaggerated, analogous color schemes (especially in his rose and blue periods)