Can we enamel every bath/shower tub?

Yes, except those in acrylic, those can be polished.

In every color?

Yes, but only solid color (no metal effect).

Why enamel an old bath/shower tub?

The price of a anamelling is much lower than that of a replacement, quick execution (about 5h), no dirt,

no disassembly necessary.

How maintain correctly my bath/shower tub?

Ideally dry your bath/shower tub after each use for no limestone trail, use a smooth 

cloth for washing, no javel, vinegar and abrasive soap.