Well you have a whole range of color open to you. Look for towels that you like and then get a rug to match.

I’d buy white towels with a colorful monogram. You could do multiple colors but the same monogram style. White is easy to bleach, and you can almost always find quality towels on sale somewhere, when they are all white you don’t notice they might not be made by the same manufacturer and you can easily replace one at a time when they eventually wear out

I am feeling red and a pale yellow. They obviously pair well with black and white, but also look lovely with grey. I like the idea of white towels since they are easy to clean, I just don't like how bleach sometimes affects the quality of the towel after several washes.

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I'd put a fluffy white rug on the floor and hang gray towels. If you opt for a white rug, buy two.

If the floor is cold in winter, consider a black or light gray gel pad beneath whatever rug you choose. Year round, you could hang a (white_ rug over the side of the tub when not in use to help keep it clean but leave the gel mat in place.

I'm looking at the flowers on the photo windowsill and LOVE the orange/red. If you like bright & cheery, I'd go for that. If you want a more subtle look, some combo of grey/white/black will work.

If you indeed like color and pattern, go for it! Everything goes with black and white. Please don't be boring and get white towels and rug. How about a stripe for the towels? Or a contrast band on a solid color? Maybe an ombre effect rug. I do use light colors mostly in my own bath, light blue and sandy color, not for ease of washing, but because they go with my shower and floor. You can get any color perfectly clean without bleach by using an oxygen cleaner or vinegar in the wash. Less harsh on the fibers.