Off the top of my head, I don't recall any description of the Janizars.
FWIW, and take with a massive grain of salt or two-thousand, the
40K Wiki gives their colors as "Chrome helmets with scarlet; cream greatcoats with gold frogging" Unsourced, of course, so again, may not be worth the pixels it's illuminated with.
I suppose you could use one of these Ottoman (non-Janissary) uniforms for inspiration (pretend they're cream...you've got greatcoat and gold frogging. Can't have everything!):