I'm making a birthday cake for my son.. I am a complete newbie when it comes to cake decorating..I made the classic mistake of running out of buttercream...I thought I had plenty, but I'm doing a buttercream transfer and I probably made a bit to much of some of the colors. I have the crumb coat done on the cake. I will have a short window of time to finish the cake Tommorow between work and the party. My question is....if I get some canned buttercream and dye it as close as possible, will it taste all right over my homemade buttercream? 

Sure, it will be fine.  You could add a little more flavoring to it to make it taste better but that is completely up to you.  

It will be fine.

Aawwwww sorry about your problem.   As the others said, you will be just fine.   You can keep the extra colors for any future cakes you may make.  Just put it in a glastic bag or container and freeze.  To use, defrost (best way: over night in frig), stir up well and use :)