How do I get the rest of the stars in super mario galaxy?

I have 99 stars, beat the game, have luigi, and i have one green star in the kitchen world. How and where do I find the rest, from what sites have said theres like purple coin stars and red stars and green stars or something??How do I get the rest of the stars in super mario galaxy?
I am pretty sure that on most galaxies there are six stars to get, with a couple only having one. If you use this as a guideline, you should be able to work out to go next. Some stars can only be found if you select a star you have already got - look for a exclamation mark (or it might be a question mark, I can't remember) above a star you have already obtained. To answer your bit about the red star, go into the little house on its own floating platform. Finally, the purple coins come from a purple coin comet in orbit. You must have come across them already, but I'll go over it anyway. If you see a extra fiery object on a galaxy, then that means it has a comet. This means it has a new star (usually a normal star repeated in a special way).Try to get the star immediately as the comets come and go, but after you have it you'll be able to play it anytime.