Here it is more often referred to as Kensington, made popular by the Kensington School for Girls in the 1880s which as you say was done in red then blue, I believe it is more about the style of stiches as well, I understand that backstitch and outline stitches were the only ones used originally. I remember reading somewhere that red was used because it was a strong colour and less likely to fade or bleed. I have also seen more modern pieces of redwork/kensington done in various colours. As to who made the rule about colour - don't know, think it was red from a purely practical point of view - interesting to see what others have to say