And since the PC is usually not on the COMPONENT my bet is you didn't connect the 3 video connections for COMPONENT VIDEO but are trying the single COMPOSITE VIDEO.
Nothing seems wrong here.
I have connected a DVD player to the monitor, using the red, white and yellow cables, but on playing a DVD, whilst the sound functions well, the picture is grainy and only black and white (no colour).
Connecting a different DVD player has the same result and the monitor works perfectly well in colour when connected to a PC. Please advise the solution; thanks.