It's impossible. Most chemicals of interest are pretty boring in the visible spectrum. I'd say >95% of pure substances I've worked with - everything from pestic

It's impossible. Most chemicals of interest are pretty boring in the visible spectrum. I'd say >95% of pure substances I've worked with - everything from pesticides to pharmaceuticals - are some variant on "white to off-white solid" or "clear to amber liquid." White/clear indicates that all photons visible to us interact with the materially equally. You get tans, yellows, and browns largely from high-frequency (deep blue/purple part of the spectrum) being absorbed by assorted chemical bonds. Spectroscopy is predominantly done with UV (200-280 nm most common) and IR, which are regions where photonic interaction is dominated by electronic and vibration/rotational transitions, respectively. Visible light absorption is typically caused by highly conjugated bonds and metal-coordination complexes. In terms of day-to-day, this is almost exclusively dyes (synthetic and natural). Dyes also tend to be really potent absorbers - you only need minuscule amounts of them to create very vivid colors. So a purely visible-light-based app would at best be able to give you a handle of what sort of dyes are in something. It won't tell you if it has pesticides (let alone traces!) or HFCS or nutrients or what-have-you. tl;dr - no, it's not remotely possible to even detect pesticides with visible light.