What you're getting from a contact mike is an audio signal. There is no such thing as a "frequency signal".
There are several externs that perform pitch recognition from an audio signal. Miller Puckette's [fiddle~] is the granddaddy, but there are others. Try a search at MaxObjects.com for others (in general, get to know MaxObjects, it is your friend… once you're past Max/MSP 101).
[fiddle~] will give you MIDI values, which are a lot easier to work with for what you want to do. IMS, fiddle~ gives you floating-point values (not every audio signal is in tune to A=440 Hz), so you'll need to round to nearest int, then take the result modulo 12 to get a pitch class (what the psychomusicologists call 'chroma'). Then just use a lookup table to map the pitch class to the RGB values you want.
There are several ways to do each of these steps.
BTW, there are two different "Scriabin color scales" that he documented in his life time. Either he changed his mind, or there may have been a misunderstanding when he was interviewed by an English psychologist who was researching synaesthesia and wrote a report on the composer as an example of this phenomenon. Scriabin spoke no English at the time; the psychologist in question spoke neither French nor Russian. There was room for misunderstanding.