What chemical is often injected into salmon to increase the color of its flesh?

This is not injected however is used to change the colour of salmon
There's a major difference between wild salmon and farm-raised fresh salmon: without certain additives, the flesh of the farm-raised salmon would not have that familiar pinkish hue. Farm-raised flesh would be grey.

To achieve the desired color, the feed of farmed salmon is supplemented with an additive. A salmon farmer can choose the colour he or she wants the fish to be. The feed will be adjusted to help achieve that colour

http://qz.com/358811/heres-why-your-farmed-salmon-has-color-added-to-it/

Almost 100 percent of all farmed salmon is artificially colored with either canthaxanthin or astaxanthin, a process sometimes euphemistically called 'color finishing.' Responding to an ever-increasing demand for salmon--which must, however, be pink -- several major chemical companies produce canthaxanthin and astaxanthin for color finishing. Swiss chemical giant Hoffman La Roche synthetically produces canthaxanthin and an astaxanthin called Carophyll Pink from petrochemicals and provides customers with its SalmoFan -- much like an artist's color wheel but in various shades of pink--to help salmon farmers and buyers create and/or order a color that sells well."

https://en.wikipedia.org/wiki/Canthaxanthin

What about carbon monoxide?

Very interesting, t_f. Go to the bottom of p. 3 and top of p. 4 on this site:

[From opencrs.com 2007 article, no longer online]