Hi All. yesterday I iced these cookies and had them sitting under fans for hours. i knew they weren't completely dry yet but at night I like to cover them with another sheet pan and stick them in the (cold) oven. This morning I pulled the cookies out so they can continue drying and noticed dark spots on the brown icing! What gives? (the cookies were baked on Monday and iced on Tuesday; yesterday's weather was really hot and humid)
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btw, my daughter is so quick to notice that I wrote "thank you" in black instead of white like the color of chalk! oops! These are for her teachers, I wonder if they'll notice that little snafu. 
It sounds like it is humidity. I run my AC when working on cookies and have not had any spots in my icing, which used to be a problem for me sometimes. I would also leave your cookies to dry fully uncovered and see if that helps. I have heard from other people that strange things can happen if wet cookies are covered.
I assume that you have ruled out that the brown color wasn't mixed thoroughly? My other guess would be that the butter/or oil is seeping through the icing and discoloring it.
Regarding the chalk coloring, I didn't notice the writing was the wrong color until you pointed it out. I bet her teachers won't notice either. 
I have had this issue with brown, and shades of light brown specifically, more than any other color. I notice it most when I make the icing and don't let it sit to achieve the "final color".
Thank you all for the replies and suggestions! I really do need to invest in a AC for the kitchen...its frustrating when you work on cookies for days to then have them get messed up like this. I'm not sure how to cover them up, any suggestions would be helpful and greatly appreciated. Tomorrow is the last day of school so I need to have these bagged tonight or early morning.
I have had this issue with brown, and shades of light brown specifically, more than any other color. I notice it most when I make the icing and don't let it sit to achieve the "final color".
Now that you mention it, I did have a similar situation happen a few weeks ago, also with brown coloring. Hmm
I had that happen on one of the first humid days on a black cookies that I had to put decorations on top of. Unfortunately, what I had to add wasn't enough to cover the huge spots ... however, by the end of the day after adjusting the air - they dried thoroughly and all the spots were gone.
I know the spots are very noticeable to you, but the recipient will probably not notice. You could cover the brown with brown sanding sanding sugar, I think that could look cute. You could also paint the brown with luster dust.
HappyLorisBaking - those are two excellent suggestions!
Thanks again guys! I will give them a few more hours in a cooler environment and see if the spots will disappear, if not I'll try some luster dust. 
I had this happen (first and only time) on white cookies that I painted with gold and silver luster dust. The spots showed up after I painted the cookies. It was so odd! Had the cookies done early enough that after an xtra day of drying the spots went away. I put them off as "butter spots" don't know what causes it though.
I live in Australia where it is currently winter. I have had this happen quite often recently, even if I used a dehydrator or dry the cookies in a low oven. Its driving me nuts. I bake the day before, I don't stack and it doesn't happen on all the cookies or on any colours specifically.
I also find that if I leave them in the dehydrator for too long then they get blotchy dark patches too.
If you come up with any answers I would love to hear them too!
