What type of icing colors are you using? With the Wilton icing colors, you need to use quite a bit. (as in A LOT) to get a good red, and still a significant amount for a good Christmas green.
What I suggest you do is take a small amount of icing (a tablespoon or two), and see how much coloring it takes to get the shade you want.
I know that there are numerous green icing color gel pastes, so you may want to experiment with different ones. I know that Moss Green, while very pretty, isn't what I would want for a Christmas green. Kelly Green is pretty good, as is leaf green.
I have been very happy with Americolor icing colors. I seem to be able to get truer colors faster, and no toothpicks!
Another thing is that BC colors get deeper as they sit, so I try to make my icing at least a couple of hours ahead of time, or overnight, to see what color it becomes.
HTH
RedPanda
You really have to use a lot of red. Let them set for a while, they get darker after they set. Then will get even a little darker when it crusts on the cake.
The green is the same way, I will also add some blue and yellow.("Yellow and blue make green", I can still remember my teacher saying that). That makes it a pretty green.
lots of food coloring. i found that gel or paste work best, liquid will change the consisticy of you buttercream so be careful
forest green (a little) will darken the green also. i put a little flavoring in my red, because i use some much food coloring. good luck 
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