This is a pretty weak justification for what seems like it'd usually be a design gimmick. It's perfectly possible for a colour change to be accessible for the

This is a pretty weak justification for what seems like it'd usually be a design gimmick. It's perfectly possible for a colour change to be accessible for the colour blind, if you do more than just modify the hue. (Extreme case: change white-on-black to black-on-white. Or more generally, swapping foreground and background colours would usually be adequate, unless there's a serious lack of contrast in the first place.) Or add an underline, or a border around the element. Point is, there are safer options than font-weight; changing weight is pretty fragile unless you tightly control other aspects of the design/content.