I definitely have warm undertones to my skin. If Brendan and I put our arms next to eachother, he looks super pink and I look really yellow. But I have very light skin, so maybe it's very very light olive, I'm not sure (my dad has olive skin and my mom has very fair skin). I have to wear warm colors. Some cool colors are okay on me, but they don't really do anything for me. They are much better if they have a warm base to them (instead of bright white, cream, and instead of kelly green, olive green, etc). And I can and do wear yellow, but it needs to be a warm sunshine yellow, anything too pale or too green and I look terrible, as AJ said. If you have warm skintone, definitely give yellow a try, you just need to find the right one!
When Bobby first did my highlights, I told him I needed a warm golden blonde, because anything ashy looks fake and washes me out. So San, if cool colors look best on you (and from what I've seen that looks to be the case), you might want to ask for an Ash blonde, or Champagne highlights/color, something with a cooler base. That would look much more natural. I like Shiny's idea of neutral highlights, but I've never heard of those before. Interesting!
Before I cut my hair, I really tried to avoid black, because it felt too strong and like too much was going on with all my hair, and the red lipstick and BLACK. But, when I went out yesterday and tried on a lot of clothes to kind of get a feel of what to look for with my new cut, I found that black didn't overwhelm me the way it used to. It still looks best paired with a more flattering color closer to my face though, as Shiny mentioned. So I bought *a* black cardigan (I'm not going to go crazy on the black).
Also, if I'm wearing a bright color in my outfit, I've found the red lipstick is too bold now with the short hair. Maybe because my face is much more noticeable, I'm not sure. I have to really blot blot blot and put a gloss over it to soften it. That kind of made me sad because I really love my red lipstick. But maybe since my hair will look the same everyday, some lipstick variety will be fun.
One last thing--has anyone ever had "their colors done?" That seasonal color thing? I read a book about it, and I figured out I am a "Spring," which is warm with light hair and light eyes. I found it mildly helpful, but I wouldn't follow it to the letter. There were some colors from other seasons that I've found I can wear, so it's a good starting place if you really have no clue about what colors you look best in. BUT, what's really helped me is just trial and error--going out, and trying stuff on with a friend who is good with color.