Transparent color vs rgba(0,0,0,0)

Behaviour is exactly the same, but transparent is compatible also with IE8. RGBA is more advanced (but lacks IE8 support) and allows you to quickly modify it, in case you would like an "almost transparent" color, without need to completely change attribute.

For example, it could be very quick to set

background-color: rgba(0,0,0,0.1);

Due to default behaviour of browsers that ignored unrecognized properties, is possible to combine them in order to use new one in newer browsers, but leave a fallback for older ones, simply typing both of them:

background-color: transparent;
background-color: rgba(0,0,0,0);

Or, more useful, in case of alreasy cited almost transparent backgrounds, you can write:

background-color: transparent;
background-color: rgba(0,0,0,0.1);

New browsers will set rgba(0,0,0,0.1) as background, overriding previous transparent declaration, but IE8 will set transparent as background, because it will ignore unrecognized rgba() value, so a slightly different result but in according to Graceful Degradation principle.