When I make buttercream I use the Wilton recipe from the Wilton Method classes. Well I also use the Walmart brand shortening....and when I color my buttercream it's confluent when its mixed. However as the buttercream sits it's like the color starts to separate and the buttercream gets spots in it.  For example I just colored one pink the other day and as it sat it started to get little white dots all over it. I've done a purple before too and it got greenish-blue colored spots and I don't understand why this is happening! Any answers??

Two questions....

1.  Does the walmart shortening say "vegetable shortening?"  I ask that because one time I accidentally bought some other weird Great Value stuff...

2. Are you creaming the shortening and butter until they are as smooth as sour cream?   This takes awhile. 

I would also ask what kind of gel coloring you're using...

I usually use the Wilton brand colors. Although I also have the AmeriColor but I've noticed it with both.

Yes, it's all veg shortening and I thought I creamed it well, but I creamed it even longer today so I guess I'll see what happens. And when you say it takes a while how long do you think? I usually let them cream for a couple min. But today I may have done like 10 min.

Do you use salt?  I had this happen a while back and spend loads of time searching for why.  Several posts here suggested it may be the salt.  I quit using salt and haven't had that problem since.   

I'm not sure exactly how long..I don't time it but it could very well be 10-15 minutes I let my mixer cream my shortening.  

I only use Americolor..but I'm sure in the beginning I used Wilton.   I've never had this happen.  

How old are your colors?   I've had my hot pink gel get spotty and I tossed it.  But I could see the spots in the gel itself. 

Interesting...I'm clueless

Salt enhances the flavor in so many things and this could be your culprit.  I always melt my salt amount in about 2 T of very warm water and let it sit as I am making my icing. It will be dissolved when it comes time to add your liquid and you should not have any problems with dot of light color.