Can you paint over this with another color?

Hello, Yes, you can paint over the black shielding paint with another colour.

Will this paint work on a basement ceiling to shield against EMF from electrical wiring and copper water pipes that is coming through to upstairs

Electrical wiring and metal water pipes are typically a source of magnetic fields NOT radio frequency radiation therefore I would not recommend this paint as a solution for this. You can eliminate the EMFs on your water pipe by having your plumber install a dielectric union and then work with an electrician to eliminate wiring errors, see http://www.electricsense.com/11420/detect-electrical-wiring-errors/

will I get cell signal?

This paint has good radio frequency shielding performance. If used properly (typically you would need to create a Faraday cage type enclosure) you will not get a signal. Ideally you want to take RF measurements before shielding, shield, then measure again, then possibly shield again. See http://www.electricsense.com/1299/cell-phone-tower-protection-tips-what-to-do-if-a-cell-phone-tower-is-erected-next-to-your-home/