mil Dress

You don't get to control what colors people not in the bridal party wear. Let her wear whatever she is comfortable in as long as it matches the formality of the wedding. But obviously not ivory or white. I feel like the more you push against the idea of red or yellow the more likely she is to show up in ivory or white. This wouldn't be a battle I would fight.

I understand you don't like the color, but I personally would rather her in red or yellow than showing up in white/ivory. I don't think the family needs to match or be in the same colors as the wedding party.

The photographer likely can dim or change the color in the pictures. Some high intensity colors may be easier to alter than basic colors. You can blame the results on the photographer's technique if your FMIL wonders about the color. Also, do you know what type of pictures you will want to display most? You may just have certain pictures that you want to see all the time and some when you want to remember more details of your special day.

I wouldn't worry too much about what she wears. Pics with in-laws are very few and often look like: 1) couple with both sets of parents, and 2) all family. You can give her a purple corsage to tie her to everyone else in the group photo. Aging mothers and their vanity tend to be fragile at these momentous occasions. I'd just concentrate on taking photos with your own mother.