Right, seems like I was partly wrong, if it's an obligatory light it needs to be white or amber.
But if it's a optional light, which a DRL is on a motorcycle this applies:
20. Optional lamps, reflectors, rear markings and devices
[ Subject to regulation 20A(9), every optional lamp ] 1 , reflector, rear marking or device fitted to a vehicle, being of a type specified in an item in column 2 of the Table below, shall comply with the provisions shown in column 3 of that Table.
TABLE
Item No. Type of lamp, reflector, rear marking or device Provisions with which compliance is required
2 Dim-dip device and running lamp Schedule 3, Part II and Parts I of Schedules 2 to 5, 7, 9 to 13 and 17 to 21 to the extent specified in Parts II of those Schedules.
Schedule 3 part II
PART II
REQUIREMENTS RELATING TO OPTIONAL DIM-DIP DEVICES AND RUNNING LAMPS
There is no requirement relating to an optional dim-dip device or an optional running lamp.