We always used the tip of a spatula in marmite and boiling water, gives the perfect colour and adds a nice smell to it that the students can't work out, so certainly adds to the "oh my God maybe this is real". Never used it with colorimetry pracs so don't know if it will have an effect.
Thanks all, I'm going to leave the colouring this time around as I haven't got time to trial how it will affect the colorimetry results.
To answer your question all you have to do is scan the whole spectrum. Since absorbance is additive all you have to do is either add or subtract the values.For fake urine I add a tea bag to cold water and stir until the desired colour is achieved. No idea whether this is suitable for colorimetry though.