Just a plain, navy blue t-shirt with khaki chinos, for example, just as a casual outfit.
Responses (1)
Of course it is. There really are no rules you need stick to where clothing is concerned, beyond your country's decency laws. As long as you are covering up all the bits that should be, and not wearing something /morally/ tasteless and inflammatory (a t-shirt with a racist slogan, for instance), then you are OK and need not listen to anyone saying otherwise.