It seems that rather than being able to go 'oh I'll have the white kitty with the cut pink gem on it's collar', that your form is actually linked to your toon's physical appearance. Yes, you can easily change how your bear or cat form will look but the only was to do this, in my understanding, is by visiting a barber shop and changing the skin tone (and/or hair) of your Druid in humanoid form.
Zarhym puts it better than I:
"Now that everyone has had a chance to check out the new art for druid cat and bear forms, we'd like to share with you the current break down of Tauren skin tones and Night Elf hair colors associated with each form color. Please note that, while we have attempted to associate each form color as carefully as possible, this list is subject to change before the next major content patch goes live. All options for skin tone (Tauren) and hair color (Night Elves) are listed below followed by the forms that will be associated with each set of tones/colors."
Click on the jump to see what your druid is going to look like.