Which colour would be good? I tried blue but the whole thing gets too 'blueish'...
Just with a quick look at the blue gradient you've used, I had these ideas. You would probably have to play around with the hue to get them to blend better.
You could try; 196,115,49
Still got the yellowish look without being to harsh with the other colours
or 196,188,49
Bright - but blends with the blues
or 8,104,100
The green should take away from being to much blue
or 196,49,56
Makes a good contrast