I agree with Orin. Is is an effect of husbandry issues. How the insects are kept can cause a bacterial issue. It is an advantageous infection of a bacteria in the (Serratia sp.?) genus. The reddish color is from the colonization of the bacteria. The best way to avoid it is to keep the colony really clean for a while. This includes removing uneaten food, not letting the enclosure get too damp, washing your hands before and after working / handling the roaches, and feeding them fresh food not partially rotten veggies etc. The most common source of a bacterial or fungal infection for a roach is from human skin.....If there is a really serious case in the colony, clean the bin, use new substrate, clean or replace the objects they climb on, and destroy any corpses or live individuals with reddish color so other roaches do not nibble on infected tissue. The remaining roaches should recover very quickly.