What do red minus % mean against values of weapons, rings etc?

It simply means that your current equipped armour or weapon or ring or pet have lower ability than that of equipment you have selected inside inventory or shop. e.g. (1)Light dagger (equipped):- 10% jump and 10% speed. (2)Poisoned dagger (selected):- 4% jump (-6% red color) and 20% speed (+10% green color). It means that you will get 6% lower jump impulse while 10% higher character speed. I hope this help you get better understanding.