I have no experience with Better Crocker coloring, but my guess is it may not be as stable as more concentrated gel colorings specifically used for iced cookies, like AmeriColor or Chefmaster. I also ice my cookies with icing mixed (with coloring) fresh that day; again, the coloring seems to set more stably if used right away and not refrigerated overnight. 100F may be too hot for cookies; I put mine IN FRONT of a dehydrator set at 90-95F, so the temperature they feel is a lot less that 100F. You'll have to watch your cookies like a hawk in the oven to make sure the icing doesn't overheat, over-inflate, and then crack. A heat fan or dehydrator with lower temperature settings are far better options. Got to run - as on major deadlines - but I hope this info helps take your cookies over this top this holiday season!
P.S. I don't know how you're able to mix my icing well by hand; it's thick to start and I could never bring it properly together without a stand mixer. It really needs a stand mixer to properly homogenize it to start.