It sounds like blue and green together are being used to affect reflectiveness, which is why cyan gives the strongest effect - in any case it seems like red is not used for shininess purposes, so Separate RGB can be used to grab the other two channels.
You could use a ColorRamp to adjust the strength of the effect before sending it the material, I guess by reducing the roughness and maybe increasing the metallicity. I suspect that the green and blue channels might be used to control two different aspects of the material but you might just have to play around until you find something that looks decent.
I've updated my Example .blend file to send the result of adding blue+green to Metallic, and then inverted it before sending it to Roughness (so cyan is inverted to black, making it 0 roughness).
