i have just painted my bathroom with a coulour i had mixed, its a vibrant emerald green , but looks kinda 'flat' can i use a paint effect to make it more interesting ?

yes!! you can!!
have you seen the pearescent glazes available in DIY stores? they come in a smallish tin, and you just paint over the green to leave a shimmer, very nice!! if you alternate the brush strokes, (like a colourwash action) it will add texture as well.
or you could add another colour over the top as a colourwash, or paint another colour in blocks or stripes. the list is endless!! i definatly like the pearl paint tho, and i dont think it is that expensive. dont use too much on the brush or roller, and it will go far!! thin coats are better than thick running ones!!
best of luck!
Riah