Although Aaronut is entirely right in stating that you would be better off using alternate colourings that do not damage the texture of your icing, you can use normal food colourings, I do quite often.
When I use them on fondant or marzipan, the main issue is the capacity of the icing to absorb the colouring. They have limited ability to tolerate the liquid, so if you need a deep colour, expect a bit of a fight.
The best method I have come up with, is to treat it like pulling candy. Put the required amount of icing in a bowl and make a well and enough material to fold over that well. Put in a small amount of colouring and fold over. I tend to start off by squidging it about in folding motions to get the liquid in to the material, it'll be quite tacky at first. Then I roll it between my hands in to a sausage, fold in half lengthways, roll, fold, roll, fold...
It'll marble up to start with, given the folds, but eventually become one solid colour and will become less tacky with each fold, in a similar way that kneading bread makes it progressively more coherent.
Once settled, roll towards a ball/block rather than a sausage, then you can roll it out ready for use. If you are a very minute amount over the moisture level that you can roll it, try to incorporate a small amount of icing sugar (confectioner's) whilst folding to dry it out, like adding flour to bread.
It works, but is messy, take all rings off and expect to need to wash your hands a lot.
Edit
Found a video that roughly shows how I would do it, except the demonstrator has a different way of applying the colouring to the fondant initially and is using a much larger quantity than I tend to, so does not hand roll in the air as much as I would or as fast.
http://www.ehow.com/video_2333485_coloring-cake-fondant.html
Buttercream is pretty simple, start with room temperature butter, beat it until it's white, slowly add icing sugar, add small amounts of milk as needed to keep the consistency right.
For each 1/4 cup (1/2 stick) of butter, use about 1 1/2 cups of icing sugar (sifted) and 3 Tbsp of milk. You can do it in a mixer, a food processor, or just use a bowl and a whisk. I used a whisk a few days ago when I realized my large stand mixer is overkill for the small amount of icing I wanted and I found that microwaving the icing for about 10 seconds helped me stir in the sugar when it was getting too thick.
Best Answer
I think you'll be fine. I had some buttercream frosting left over from answering a previous question and did a simple test not long after you asked this question (about 16 hours ago).
I put a dish of frosting into the fridge with small royal icing and sugar/starch based decorations on top and I left a similar dish at room temperature. I pushed the decorations firmly into the frosting.
The decorations retained pretty much all their crunch. They didn't leak any of their bright coloring into the frosting either; not that that would be a problem if you're decorations are snowflakes. I'm not sure whether the opposite would be a problem, i.e. if your frosting is brightly colored and your decorations are white.
You can judge for yourself whether the ingredient proportions I tested with are comparable to yours by checking the answer I linked to above. You may see different results if the proportion of wet ingredients in your recipe is much larger.