"If your horse turned all grey I dont think you would enter the multi-colour class when that happened but until then you could."
Just had to add this - I was showing at a fun show with a horse that had grayed to pure white, but was born sorrel pinto. The judge was a friend. I put my horse in multi color, and she walked around my horse about 6 times, even bending down to check under the belly, jaw, neck, and mane. Finally she stepped up to me and asked "why is this horse in this class?" Because he's a pinto!! Although you can only see his spots when he's wet!!
So if this is a graying horse - turning white all over - then you might not want to put him in a color class and hope to do well, because he would have to show in multi color. But, if you are just qualifying for Nationals, it's a quick class with much less stress.
Now if he is a silver dapple and white, where the colors are clearly defined, then absolutely you can put him in. Lots of judges like silver dapple pintos.