I have no idea what's in Color Flow, so I can't comment on how it differs from meringue powder, but . . .
Cream of tartar is usually added in very small quantity to egg foams/meringue (which can be made with meringue powder, of course) to stabilize the egg foam. Its function in royal icing might be some small amount of added stability, but because royal icing is usually mixed more thickly and is less aerated than meringue, I don't think the need for such stabilization is as critical. I've made royal icing without it and it's behaved pretty much as it always does.
Cream of tartar is also acidic and its often added to baked goods to activate baking soda and turn it into baking powder.
You can read more about the function of cream of tartar here (http://www.thekitchn.com/food-...at-is-cream-of-72125). Liesbet, our Toolbox Talk contributor, will also be doing an extensive post about cream of tartar this month, and I'll see if she's up for tackling some of your other questions.
This all said, I'm SUPER curious if you tasted your icing - adding 1 TB of cream of tartar to 3 TB of anything is a gargantuan amount (I usually add 1/2 tsp to 2 lb of powdered sugar and 5 egg whites to make royal icing), and I'd be surprised if the icing wasn't very tart.