I can't upload a picture or else I would attach one
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You haven't received an answer, so I'll ask. What is your recipe and is the icing on cookies? Can you post your recipe and share more information?
4 cups icing sugar
2 tablespoons meringue powder
1 teaspoon vanilla
1/4 cup water
It happens the day after I flood the cookies (I dunk them in a bowl of icing and flip over to remove the excess icing from the edges)
I checked Cake Central website, this is the info I found. I don't know if it helps, but I hope it does.
It's a problem that has been asked on CC by members in the past. It could be a number of issues:
1) did you use a large amount of food colouring? Dark colour? Too much and it can crystallize/have dark patches. (For those dark/intense colours you need to tint, use Americolor. It's much more concentrated than other brands like Wilton & CakeCraft.)
2) did you use too much water to thin out the icing? A high concentration/ratio of water/colouring can also cause this.
3) did you allow the icing to mix properly, esp. after thinning it with water?
4) is the area where the cookies are drying humid? unevenly heated/cooling?
5) did a drop of oil/butter/fats get into your icing while it mixed? Were your paddle/bowl/piping bags/tips completely clean and oil-free before you began?
Read more at http://www.cakecentral.com/forum/t/655553/royal-icing-changing-color-as-it-dries-on-the-cookie#a70g70helmo67UzA.99
This was very helpful! Thank you so much :)