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I just had a quick google and could only find info stating the front light must be white
You must have a white front light and a red rear when riding after dark, but you can have additional lights in other colours as long as you don't show white to the rear or red to the front. If you show blue lights, you may have to convince Mr Plod that it isn't, and doesn't resemble, "a blue warning beacon or special warning lamp"
I just had a quick google and could only find info stating the front light must be white. Blue, green etc colours are for emegrency vehicles only (maybe this doesn't apply to bicycles?).
I couldn't find the exact legislation though.