If it were me and I were painting the stock Tamiya cupola I would go with a semi-gloss black.

If using a clear lense I would check to see what it looked like left alone, then maybe a dark blue with some black added to it. The lense used is different in outward appearance as to the periscope lenses (from WWII) which appears as green or bluish tint but the fact remains... do what looks good to your eyes or matching up to supplied images.




Jeff