Red Green and Blue RCA's are commonly mis-labled RGB - this is incorrect. RGB is a connection used by predominantly within a scart connection, and differs in specification than Red Green and Blue phono connections, which is actually YUV, also known as two varients - YPrPb and YCrCb - the former being analogue and the latter being digital. RGB over scart differs from YUV in that it has separate pins for red, blue and green, aswell as a 4th pin for sync, whereas YUV has dedicated red and blue, and sync is part of the green connection (known as "sync on green") Chromience and Luminance are lumped into the other two connections, whereas RGB over scart has dedicated pins for this. To demonstrate how shit composite (yellow video RCA) is, all of the above (red, blue, green, sync, chromience and luminance are all done using one cable core and one "pin" connection. Poor indeed.......

Scart, in which ever form (beit RGB, S-Video or Composite) cannot carry anything over it's maximum of 576i standard def picture. YUV has the bandwidth to be able to carry both 576i SD aswell as up to 1080p HD, along with other varients along the way such as 480p (SD) 576p (SD) 720p (HD) and 1080i (HD)

The ability to transmit HD over component (YUV) is down to the permissions of the hardware - the TV has to be able to accept HD over YUV and the source unit has to be able to output HD over YUV. Most of the time this is in the box settings with a simple on/off toggle.

As to which is better, HDMI or YUV, in many cases it is down to personal preference. In many cases, usually with cheaper components, HDMI is seen to be sharper, and clearer, but at the same time not as warm or vibrant. YUV can come off slightly "softer" (also reported as "not as harsh") but often with warmer and more vibrant colours/deeper colours. HDMI is however, a fully digital transmission (as in binary - 1's and 0's) and also carries the ability to transmit upto 8 channels of uncompressed (HD) audio along the same cable as it's 1080p (HD) video.