Stacy 3C, my mother was overly critical of my hair, too. I think she hated every hairstyle or cut I ever had, and was always making comments about it. She didn't overly criticize other aspects of my life, so I'm not sure why the hair was always such an issue.
One time shortly after I was married, she came to visit me and I hadn't seen her in 2-3 months, so I went and got my hair cut the day before she arrived. Before she even got in the house, she said "you need a haircut". <sigh> It's hard not to take it personally, but i think maybe she just didn't get that current hairstyles don't look like "good" hairstyles in her generation... she was in her 40's when i was born, and as far back as I can remember she always had the "little old lady poof-head" style, done in a salon weekly.
Still, it is hard not to take it personally! But don't give in and color because she said to. Whatever her issue is with your hair, it's about her - not you. So if you color just do it because YOU want to.
(eek, sorry I got so long-winded. moms + hair is apparently still a sore subject with me!)