I have had the same thing happen to me with both light and dark colors, the dark colors are more obvious. I researched, and researched and came up with humidity and butter bleed. That said, I changed my sugar cookie recipe to one with less butter and invested in a quality dehydrator. I haven't seen the issue since. If you can't afford to invest in a quality dehydrator, a cheap alternative is to place your cookies in a cold oven with the oven light on for a few hours. Place your cookies a few racks down from the light. As a previous poster noted, avoid placing your cookies in a covered container immediately after icing. I don't pretend to be an expert, but it seems reasonable that covering your cookies before the icing has completely dried may trap moisture within the container possibly contributing to the spotting if it is indeed caused by humidity? I hope these tips help.