I would suggest painting it a lighter color on the same paint card to avoid the greens clashing.

I think it's young that's all. I love mixing greens, as just this border shows. You could mix the 2 with shower curtain, etc... what do they say about the paints?

Wallpaper Border - Green Live in Peace Wallpaper Border, Prepasted · More Info

If you wanted a lighter green - would it 'stand up to' the strength of the other [which from here looks pretty strong]?? would it look blah by comparison? When you add white tile won't there be less of the green....

Have you thought about painting the ceilings in both rooms the same color? Not green...thinking something like this:

course you can change paint - but a curtain.

Sweet Jojo Designs Turquoise and Lime Layla Shower Curtain · More Info

Oh, the 2nd photo is the 'muddy' muted color you're not sure about - is that correct? I thought the other way.

I like idea of using same company. One time I chose about same saturation/ level on the card, but a few over -- I don't know why you couldn't mix that way.

[https://www.houzz.com/magazine/how-to-pick-the-right-green-paint-stsetivw-vs~462325[(https://www.houzz.com/magazine/how-to-pick-the-right-green-paint-stsetivw-vs~462325)

Maybe paint them both the clear green - can use accessories in lime for sure.

Sorry one more, but often you'll see soft lime and clear aqua together [yellow-green with blue-green]:

Set of 3 Long and Uniquely Designed Ceramic Vases · More Info

Depends, but those 3 might be fairly strong in impact.... maybe a softer blue: cornflower or...

?

Pitch Blue Estate Eggshell Paint · More Info

Not sure - post flooring photo....and what's the flooring in its hallway?

You called the paint muddy - from expert MK: Color can only be described in three ways, it is ALWAYS either: 1) Clean or Dirty 2) Light or Dark 3) Cool or Warm... it's quite the study that I haven't yet done. Have you changed the other room?

I would suggest a light coral color for the bathroom so the room is crisp and bright like the mint bedroom. You can then incorporate a little color of each room into the other through accessories to tie them together.

Dev18, I didn't realize it was a boys bathroom since the original post just said kids. I've actually seen orange used more than mint for boys' rooms. You can make the decor accessories in an orange room fit for boys. You can also make the orange color a little more intense. A mint/orange/gray color scheme as seen in the shower curtain below may be a good option. Orange and gray could add a more boyish feel to the mint green.