Not true. There is such a thing, very much so in fact.

The full size use multiple injectors, one for the basic white, and the others to inject the various color dyes (look at a red arrow hawk from behind, you will see 3 tubes). The reds use a textile dye on biological basis, not sure on other teams, but i'm sure they have similar stuff....you can get it from just about every textile shop around.

As to models: it's been done a lot. The Phantomstaffel had it a while, Stephan Völker apparently had it going in south africa from what i heard....they all did not stick to it as it was a huge mess on the plane, in cars, on your hands and everywhere the smoke got (all over the airport). Peter Mayer of jetwelt apparently sells the stuff in red, blue and yellow in a single-injection ready to go system from what i recollect. Not as colorful as if it were injected seperately, but less technical stuff to carry along :-)

Best regards
Hank