It is from the nominal sense of color meaning “evocative description” that is commonly used in journalism. The first cites in the OED:
1938 E. WAUGH Scoop v. 87 We’re paid to supply news [...] Of course there’s colour. Colour is just a lot of bull’s-eyes about nothing. Ibid. 89 They gave Jakes the Nobel Peace Prize for his harrowing descriptions of the carnage—but that was colour stuff. 1953 BERG Dict. New Words (ed. 2) 57/2 Colour, in a programme: subsidiary features added to evoke, or enhance interest; e.g. fifteen minutes of colour before the kick-off.
This comes from the more general sense meaning character or a figurative complexion, which goes back to Shakespeare.