So yesterday I whipped up a couple of batches of Sugarshack's buttercream and tinted the colors I needed. I iced my cake last night and this morning it's speckled!!! 
I used Wilton colors, which maybe could be the reason. I was just wondering if anyone else has had this happen and if they know what the reason is. The color I used was teal. Did I not get it mixed well enough? The cake was stored overnight underneath a cake save that was not tightly sealed.
This used to happen to me as well.
Did you use salt? I've gotten into the habit of adding the salt at the same time as I add my liquid (before dry ingredients). This has stopped the specks. If you're not using salt (or even if you are), I would try letting it blend a little longer so everything has a chance to cream together well. See if that helps.
Are the colors darker or lighter?
If they are darker spots you may not have gotten the color blended in all the way.
If they are lighter spots, well I don't think it is salt myself because I have had too many students with dots in their icing (purple has blue dots, pink and blue will get white dots) and most of the time they did not add the salt to the icing. I actually believe it is a reaction to the chemicals/minerals in the water.
You can use bottled water to see if that helps, but the good news for you is once the icing has spotted all you have to do is stir it up and you are good to go. It will not respot. So you acutally did a good thing by making your icing and coloring it ahead of time. 
I have gotten speckled icing if the shortening I used didnt have any transfats... just a thought.
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