White shirt is something which you can wear formal or causal and it never out of trend. White is color which is good for any event you wish to attend.
You can wear white shirt with blue denim always I can swear by this combination.
But you can wear them with back denim or trousers also with grey or dark blue denim/ trousers.
I will look very classy with light brown/ colored trousers also dark or army green color trousers will go.