I recently finished a book wherein the girl's hair kept changing from brown to blonde. No, she wasn't auditioning to be the new Clairol girl -- the author messed up and the editor didn't catch it. Mistakes like this have three effects:
1. It makes the author look like a doofus, which is never good.
2. It throws the reader out of the story.
3. It keeps the reader from forming an image in their mind of what the character looks like, and so the reader isn't able to bond with that character, and they don't get the full benefit of the story.
Here are some tips for keeping your characters straight:
A. Keep a notebook by your computer. Every time you add a new character, take a minute to jot down some vital statistics. Then, any time you refer to a trait of that character's, glance at your list to make sure you've got the right person. You can also do this on Post-it Notes and stick 'em on your monitor. I like doing that because it makes me look productive when someone glances at my computer.
B. Look through magazines and catalogs and find pictures of actors or models that resemble your characters. Cut out those pictures and keep them for reference. Anita Stansfield does this, but most of her pictures are of Kevin Costner. Hey, we're all entitled to have our dream men, aren't we?
C. Really envision your characters as you write. Get to know them like you would a new friend. Then, as you're writing about them, the descriptions will flow naturally, as they would if you were writing about a real person or a new friend.
You should know your characters better than anyone. After all, you've just spent how many months/years thinking about them all the time?