By my reading of the Road Vehicle Lighting Regulations, the only non-white lights you can show to the front of a car in the UK are indicators and headlamps. All non-mandatory lights must confprm to the standards of the mandatory ones. From this DRLs must be white. Any light must be "E" marked and as yellow is not correct for DRLs your light will be illegal on that count too. I don't think the MOT station would fail for it, but a police officer trained in vehicle inspection might give you a ticket. The biggest issue I can see is that it will give any copper and excuse to pull you over.