I'm trying to get a light blue in which I did with the fondant but it was pure white, my buttercream is tinted yellow I'm assuming from the butter and when I put in the blue it's green(of course yellow n blue make green) what do I need to do, dont want to use all shortening like someone suggested to me. Thanks!

My 1st suggestion is to test a small amount.
Is your b/c ALL butter or part w/shortening.

The only way I know to make a light blue(which is lighter than sky blue) is to add a couple of drops of white to the blue. But I would try it with just a couple of teaspoons of the buttercream just to be on the safe side.

it's butter and crisco

On the color wheel orange is the opposite of green. Maybe try a tiny tip of a toothpick of orange and see if it helps get rid of the green tint. Just try it on a little bit of icing.

Have you tried it and got a green tint or you are assuming it will because the buttercream is made with butter?

I've always made buttercream with butter and never had trouble coloring it. But like the others said, try a small amount first, so if it doesn't work you won't ruin an entire batch of icing.