I would pick the toughest to change items first. If you HAVE to pick tile after wall color and vanity & top you need to post pics as description alone of not enough.

It's a powder room, you already have a marble vanity top, make it special and choose a beautiful marble floor tile. It doesn't take any more care than any other tile floor Unless you don't clean at all. It's the jewel box of your home.

https://www.perigold.com/Ackland--Angora-Geometric-12-x-12-Polished-Marble-Mosaic-Floor-and-Wall-Tile-WAYANGORAGEOP-L2970-K~MVP12292.html?refid=MX4260609366194-MVP12292&device=c&ptid=%7BCriterionId%7D&targetid=%7BTargetID%7D&network=%7Bnetwork%7D&refid=PSD457-mvp12292&costid=PSD457&pin_campaign=626755961981&pin_adgroup=2680086738462&pin_productgroup=4260609366194&device=desktop&pp=0&epik=dj0yJnU9NGM4emVQSTAwcms0azh6dTJnMnpvLVRXVVhwb01JZ2kmcD0xJm49YUlUZlVuT3poMWs2Zk5MdWV3M20wdyZ0PUFBQUFBR2psS1Q0

You can get a porcelain tile that looks like marble. Since they will not be next to each other they don't need to be identical, but they should be pretty similar. Or look for a gray floor tile, or a black and white patterned tile. Post a picture of what you have that you're keeping so people can give you good advice. It will help if people can see the style of the vanity.

I would do a light gray tile, maybe in a herringbone pattern.

Get samples - gray, white and marble. It’s really hard to decide without putting some tiles on the floor and seeing how they look. It’s not just a color choice. Tiles come in a lot of sizes, and can be installed in different ways, which add to their design.

Tilebar sells 5 samples for $5. You have to see tiles in your bathroom to assess which is best.

BM Quiet Moments is one of the most beautiful pint colors out there. Gorgeous choice for the walls.