I would use grout paint to lighten the grout here, which would update it a bit -- if you go a bit warmer with the gray, or toward taupe, that could solve your color problem. Of course, it's also true that the wall color in a bathroom doesn't need to match the walls in the bedroom, so you could do any color scheme in here that goes with your future plans, or wait until you get around to updating this space, and stick with the pale yellow for now. White floor tile shouldn't limit your other choices much at all.

I think the rod should be about 4 feet wide gives you about 6 inches on each side so you can pull curtains to the side. One curtain could work if it was wide enough (4' if you go with that rod size) but would not look good IMO. If you are leaving that shade in the window you could maybe go with a dark grey or dark blue curtain instead of a black out and it would still darken. for height you have to figure how much space is below the window and account for the rod being mounted higher than the window. guessing from the picture you will probably need 95" inch long curtains

A quick Google found these - https://www.homedepot.com/b/Window-Treatments-Curtains/Grommet/1215/N-5yc1vZarfwZ1z0z5ilZ1z1a3uj But I wasn't able to find other stores that list any longer than 120". re: your narrow corner window, depending on the panels you find and their width, you may be able to cut a panel in half length-wide to use on that window. If you or a friend sew, it would be easy to fold over and then stitch up the cut ends. Do you want to be able to close the drapes fully over the windows, or are you adding them just at each end for color?

I love Solera Roman Shades from Hunter Douglas. I have them in all three of my kids rooms. They can be cordless, and they offer energy efficiency. The benefit is that the shape of the pleats will just pop back into position if they are fussed with. My older boys spent their young childhoods abusing the shades, and they still look beautiful. LOL. I don't think you have to do motorized–I would leave the high window shades closed. You have so many other lighting options from the other windows. And yes, I love the idea of adding curtains to the lower ones.