I recently make a cake and using wilton sky blue color and cut out fondant circles to add to the cake like they wanted. Cake looked great but the next day after the day had time to crust over it had little white spots all over the icing. At first I thought it was from maybe water sprinkles from my hands but now not sure. I will attach a pic so please feel free to view and let me know what you think happened. It also did this on my trial cake. The photo colors are distorted because I blew it up so that you can see the spots.
I've heard it can be caused by salt not being disolved. Maybe using popcorn salt or grinding regular salt finer might help.
lorrieg is correct, it sounds like your salt didn't get fully dissolved before you added your solids. I had that happen once and after that I have basically quit adding salt to my buttercream.
Actually there are two or three colors from Wilton that are known to have issues similar to this. I find that with the violet you get spots of blue, and with (I think) delphinium blue you get funny spots. I can't remember exactly all the colors we've determined have problems. You may want consider another brand of color. Americolor is good.
The later appearing white spots has been happening for many years. I don't think anyone has found the answer.
Yah, many people think it's from salt but then some say they didn't add any salt to their icing and still the spots happened.
Yrs ago I was sure it had to do w/the minerals in the local water supply as it seemed to happen to the students from the next town over but not the ones where I lived