I could tell you what I reckon would look good, but it's hard to make a good decision without actually seeing the bathroom.
If you keep the light green tiles, and the yellow tub, I suggest giving the shower walls a sky blue, and the rest of the bathroom walls a (pale?) orange colour.
Honestly, the yellow tub and light green tiles and dark green walls would probably look really kiddy, and unprofessional. Even if you got rid of the green tiles, but kept the yellow tub - which it sounds like you will - your only choices are pale colours. Light colours that don't clash, but some like pink would again, look kiddy, only girly as well. Without using dark colours, you're gonna end up having a pretty girly bathroom anyway (unless it's a very desaturated "colour").
You don't want the childish appearance, nor a blinding appearance. If you were to buy paint with the help of someone who works at the store, they're going to tell you the same sort of thing (unless they're idiots or douches).
I think, keep the yellow tub and the light green tiles, and give the bathroom a grass, sun and sky theme. Blue and orange are great contrasting colours that fit the concept of 'sky'. Daytime and sundown. Two examples of each, for whichever works best to you.
Either of these blue colours for the shower walls:


And either of these orange colours for the rest:


The blue and orange combination draws attention away from the fact that the bathtub has a bad colour, and gives the impression that the tub's colour was intentional. Hopefully it wouldn't seem crass.