We just built a house, and love Collonade Grey by Sherwin Williams that’s on our main floor. It’s very much a “greige” that takes on different hues depending on how much light a room gets, decor, etc. I’m very impressed by how warm it is for a grey, and how neutral it is. We had a designer help pick out paint and tile, and she nailed it!

Benjamin Moore sailcloth for the walls & cabinets-- a pale slightly greyed beige, very relaxing in our small bathroom, and light enough to reflect light. Ceiling in Benjamin Moore "cotton balls", a pale creamy white. Doors & trim are Super White (also BM). It makes our small bathroom with just one north-facing window feel clean and light, and is not as cold feeling as a grey or bluish color would be.

Hello, I have been happy with the grey/greens using one wall as a focal point with a slightly deeper tint of same color.
I have also used a very light robin’s egg blue and grey/blues in bedrooms and bathrooms in my home and these colors bring me joy and look great with white trim. Go well with the carpet same color as you are showing.
I find too much grey sad and uninspiring although I love it in others’ homes as it is very soothing.
Enjoy the transformation!

Sherwin William's silvermist, a pale blue grey or sofentend green a pale green grey. Done in a satin finish you cant go wrong. Both are understated and will look beautiful with your existing wall colors. I would do a chair rail in the bath with the existing color below and the new color above

Before looking at the space I had this vision of a bright teal blue and coral orange - to use one of those colors in each space, and then use both those colors in each space. So perhaps coral orange in the bedroom, teal in the bath, and orange and teal accessories in pillows, shower curtain, towels, etc. I think such rooms that are going to be used occasionally - sounds like it will be a guest suite - you can be a little wild and crazy and it all works well. And, with a neutral like grey in the hallway it will all work fine.

Another option if that seems too wild go with grey or white in the bedroom, and perhaps yellow in the bath, and carry that yellow into the bedroom. Yellow and grey go great together.

We've been too grey for too long - I rather crave pops of color...!

https://nylonliving.com/teal-and-orange-decor/

https://www.euffslemani.com/orange-and-teal-home-decor.html

https://www.digsdigs.com/grey-yellow-living-room-ideas/

I would suggest painting SW Mindful grey. It is the perfect gray (in my option) and is neutral so you can use any pop of color.

Would the mirror that's in the hallway fit over the sink in the bathroom?

If it is truly for guests try to boost the lighting as much as a help to your visitors. I would go with a shower curtain that admits some light into the shower -- a translucent or mildly transparent, and for pops of color, consider very brightly colored towels, face clothes and other necessities and niceties so they are easy to spot.

Stick with Edgecomb Gray, or try BM's Pale Oak. It can look greige or gray depending on the light.

This is a really nice room. I am thinking a pretty taupe would work well with the carpet and your hall colors. I can find a good one for you if that is of interest.

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Ignore the styling but something along this line. A deeper color is soothing to guests who have traveled and need a cozy environment. Use all white trim and possibly bedding for a fresh, crisp look.

My favorite color palette site is https://www.design-seeds.com

The color combinations are wonderful and puts together imaginative and creative color solutions.

Here's just two palettes to give you an idea. I understand you want to be able to change out bathroom accessories, but consider a crisp bright color like the bright green below, and stick with simple accessories. I love the taupe that Flo suggested above, but I'm afraid with no light back there, it may be too somber.

You will go crazy and down the rabbit hole looking at the design-seeds site. Sorry for that! One of the palettes is going to make you very happy and you will know it when you see it. Have fun and let us know how you are doing!

We recently renovated our garden level basement and wanted to use light colors to maximize light. I was worried a neutral would be boring, but I love how warm and bright our Sherwin Williams Varsity White is. Our light faux wood floor is similar to your sandy carpet.

We installed wall sconces because there isn’t room for furniture to put lamps on, but any lamps or uplights will help.

You can see the two bedrooms from the main room.

I agree with @Denise Marchand that a bright light color could be great for your low light bathroom. A blue, yellow or green can be a great background—act as a neutral for multiple color palettes so you don’t feel limited to any specific one. We painted the laundry closet a pale bright yellow that makes me so happy & maximizes the light from one small window.

@ital mover’s suggestion for a translucent shower curtain is excellent. Our only bathroom window is behind our claw foot tub, so I have translucent plastic shower curtains around the tub (to protect walls & woodwork) with a tension rod in front for more decorative semi-transparent floor to ceiling curtains to allow light through.

https://m2.ikea.com/us/en/p/borghild-sheer-curtains-1-pair-white-00291297/

Continuing the edge combe grey into the bedroom and bath would give you a brighter room and a neutral backdrop to tie the spaces together. Then your art and linens would add the color which you could easily change. You could pick a shade darker off the paint chip if you didnt want the hall, bath and bedroom all the exact same grey. Do use bright or saturated hues in decor if you paint the walls all neutral.

Just a quick tip. All colors on a paint strip aren’t necessarily compatible. A common misconception.

Have you considered wallpaper? Perhaps on one wall? Then you can pick colors from that. We just added a huge addition that was painted SW passive gray. In the Family Room and office, we used wallpaper on just one wall. In the kitchen and hallways we used white beadboard wainscoting.

I recently used Benjamin Moore Smoke in a small white low light bath and was very pleased with how it looks with beige floor tile with gray veining. Brightened up the room consideraby!

Stick to light colors as the space is not well lit and your main furniture is dark as well. You can tie in the gray from your hall with accessories that can be changed out when you chose.

I am a huge fan of BM "Quiet Moments"...the prettiest pale blue...it is currently the color of our master bedroom and I receive compliments all the time...the next darker color on the strip is "Beach Glass"...haven't used it but maybe for the bathroom walls...I think it would play very nicely with your Edgecombe Gray hallway....good luck!!!

I agree with equinekdc. One thing to look at when selecting your wall colors is the LRV rating. The higher the better for brightening a space.

Thanks for all the advice. I took the advice to choose the same color as hallway (Benjamin Moore Edgecomb Gray). I'm pleased how it turned out. I'm sharing pics for anyone who wants to see the "after" pics.

Beautiful! Very restful retreat for your guests. The Edgecomb Gray is lighter than it appeared in hallway pics. One thing I really appreciate when staying elsewhere (especially when jet-lagged) is room darkening curtains. It looks like you have room to hang a pair so they could hang outside the window moulding so as not to block light when open.