Egg White Cookies

I decided to make peppermint trees as seen on Pinterest. Here.
The Cookies are pretty and I had the basic ingredients but I had 7 egg whites to use so I did the math...
Mine look like this:

Very green, right?
There is green but it is buried under the too brown exterior.

The reasons's I failed;

• I did not follow the recipe exactly.
• I did not use the gel food coloring, I used liquid
• I was in a hurry so I turned up the heat, just a little, to 250.

Actually the cookies taste pretty good, but they sagged, possibly due to the liquid in the food coloring? Or does it really take an extra 15 minutes to beat 7 egg whites instead of 3? They are also very fragile and shatter easily. I drizzled chocolate over some and they were the best, hence none in this photo. I'll have to try again...maybe next year.