I have a nice light purple shirt with a bit of dark brown stripe in it that goes well with some dark brown pants. The pants are Brioni as well and the colour of the brown stripe matches the pant exactly. Edit - here is the pic:
It would help seeing the pants, since they are a darker color I owul go with a lighter color shirt, anything white, cream, light green, blue, yellow even pink or a peach color would look nice. Of course weather or not you choose to wear a tie or coat will also be a contibuting factor.
Colours - ivory, cream, light blue, lilac. Yellow/peach if your skin tone allows for it (mine doesn't... ). I like SoCal's idea of purple... but would worry a dark purple and dark brown trousers might be too dark... possibly with a beige sweater-vest or similar to break up the purple?
Patterns - ginghams, tattersalls and solids I think. Stripes seem a touch too formal for brown trousers to me, though I must admit to having worn them together myself too.