Yellow: Can be used to the SIDE and/or REAR
Example: VC104+ Rigid Grade, yellow
Red: Can be used to the REAR
Exmple: VC104+ Rigid Grade, red
White: Can be used to the SIDE
Example: VC104+ Rigid Grade, white

*) Note: Special marking requirements in Poland and Ireland
Poland: Only yellow to the rear
Ireland: Only red to the rear 

It is advised always to check with the National Road Authorities, for special national requirements regarding colours to be used.