Not sure about fog light bulbs, actually Eurolite makes a purple bulb ive seen, www.toucanindustries.com is where you may find them. Also www.streetglow.com has these illuminators that make them glow a color at night, similar to strobes for install i have blue ones in my fogs. My page has pics of what they look like.
Not sure where you can get them, but you do realize that anything other than white or yellow/amber would be illegal on the road. Just don't want you to find out the hard way, like I did with my purple neon license plate frame many, many years ago.
Are you looking for Performance or Show? anything other than white or amber is illegal and serve no functional purpose other than show. Colored bulbs are extremely bright if you are looking at them and look great from afar but from the drivers point of view the light dissapates in all directions and illuminates nothing.