I'm far from a genetics expert, but I'll give you my two cents. Blue, black, and splash silkies do well penned together and bred together. Many people do it. If you buy hatching eggs, you'll sometimes see them marked BBS, because blue, black, or splash may hatch from the eggs.
As Sonoran said above, it's generally best to separate colors. If they're just your pets, then you can put them all together, but if you're looking to improve quality, show, sell them, you want to try to improve.
Some of the newer colors (like paint) still aren't all the way worked out and can throw other colors. That's something that is trying to be resolved so they breed true to the color they are. By taking established colors that are recognized and breed true, and then cross them to another color, you're kind of back tracking on everything that's been done to get them where they are. But if you just want a bunch of mixed colors for fun and hobby, you certainly can.
Paints for example, are my favorite color. They are still known for throwing odd colors in offspring. Some of this is from them having been bred to other colors along the way to experiment with the paint coloration. It's taking a long time to reverse that. Sometimes paints are bred to high quality black or other colors to try to get improvements from new bloodlines, but in general, with established colors like blue and buff, you wouldn't want to mix the two for a serious project.
Hope all that makes sense!
