If the walls and colors are just too much to handle, think about maybe some curtains that hang from ceiling to floor, in maybe a darker color, like burgundy or something of that nature. The room will darken up, but you can add alot of mood lighting that way..make it into almost a goddess palace. The benefit to this is that the curtains are not permanant and no one needs to know what outrageous colors are underneath.
OR,
Try starching cool fabric onto the walls, the outcome will look somewhat like wallpaper, but not permanant, you can look online for places to do this..
the bathroom in yellow sounds cheery, i would add maybe some pretty gerber daisys in either white or a contrasting yellow, simple but cute designs...maybe get some cute postcards frame them and put them in the bathroom, simple solid color towels and shower curtains in a contrasting color to that of the yellow bathroom, pick something that will compliment it...like a cream, or hunter green, whatever your feeling when you go into it.
OK
there are my two pennies