Assuming the slabs are concrete we can recommend Coo-Var Acrylic Floor Paint, this available in flint grey.

If you require an antislip paint please see Coo-Var Anti-Slip Acrylic Floor Paint.

Smooth surfaces should be roughened first to provide a good "key" for the paint. This can be done mechanically by abrading or blasting, or chemically, using Coo-Var Acid Etch.