It depends on that, what you want to say :-)
egy alma = an apple / az alma = the apple
egy piros alma = a red apple / a piros alma = the red apple
If the adjective is following the noun, it means something else.
Egy alma piros. = One apple is red.
Az alma piros = The apple is red.
If you put the adjective on the first place of the sentence, you put the focus on that word.
Piros az alma. = The apple is RED (... and not green).
I hope, it is clear for you
I heard about the fact, that vörös would be an equvalent for the color of red vine ("bordeaux"), but I think, that's not correct at all.
A lot of red things, which might have an emotional context, are used rather with vörös than with piros. So red wine is vörös bor, Mars as the red planet is vörös bolygó, the red flag of the communism is vörös zászló as the red place in Moscow is vörös tér in Hungarian, Manfred von Richthofen called "the red baron" is in Hungarian "vörös baró" and finally red hair on the head is vörös haj.
That is partially true though, we do say that the traffic light is "piros" while blood and wine are "vörös". Usually "vörös" refers to deep red and "piros" refers to light red, but note that there is no strict rule for this. There are some examples however, where using red is obligatory, like those listed above by Andreas305.