Tell us about yourself!
I think that is a great idea, not stupid at all. There are some limitations. Because the RGB LEDs are really just 3 LEDs in the same case (red, blue, green) I think that you would want to use just those three peak wavelengths (which should be on the datasheet for the LED). The RGB LEDs make a range of colours by combining outputs from the R, G and B - but those mixtures aren't great for spectrophometry, where you want a sharp peak at the desired wavelength. Hope that makes sense and good luck
¡Cómo convertir tu smartphone en un microscopio digital por menos de $10!
How To Dovetail
Simple hardware storage bins
An Arduino-powered microplate spectrophotometer