According to the Wilton website, you can substitute meringue powder with color flow, but there will be a slight difference.
Here's what Wilton has to say:
Substitution of Color Flow Mix for Royal Icing
Color Flow Mix can be used in place of Wilton Meringue Powder to make royal icing. Follow the same recipe substituting Color Flow in same proportions. Color Flow recipe will be shinier and more brittle. Meringue Powder usage will create a lighter, fluffier icing because cream of tartar is present.
You can read some more about it here:
http://www.wilton.com/idea/Color-Flow
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