Parent and In-laws Attire

Have they brought up any suggestions to you? Usually they just wear what they feel like but if they’re asking you for suggestions of color and what not then I’d suggest something more neutral

Whatever they feel comfortable in.

Whatever is comfortable for sure. Men: dress pants, dress shirt, jacket or blazer (any of the combinations)
Woman: shift or sheath dress, skirt and blouse or dress slacks and blouse or chic jumpsuit

I don’t really think they can go wrong with what they wear since it’s semi-formal. I think as long as it’s something age appropriate (depending how old they are [imho unless you are JLO there comes a time when certain things should be tucked away]) and they’re comfortable, then you’re a ok.

At my SIL's wedding the moms were asked to where navy blue (one of the colors) and that both dresses had to be long. The dresses look amazing in the pictures. For my wedding i told my mom to where a gold sparkly long dress. She's a flashy person who loves gold and has darker skin so it compliments her very well. My FMIL skin tone looks better with silver than gold so for her i'm not sure on color yet for her dress but i want them to compliment the bridal party's dresses.

They get to pick whatever dress they like within the color and length guidelines. I'd just give them lots of time to find the dress if you are pickier so they aren't searching last minute.