Can you paint hearth tiles? and can you use them behind the fire?

Heat proof paint on the hearth should be ok, tiles behind heat not a good idea, you can buy purpose made chairbricks, but the fireplace may have to come out to fit it, there are laws regarding clearances, ventilation, hearth commissioning, flues (design, use, size and seals and commissioning), and certificates to be supplied to the effect of the above (if gas anyway, coal is similar).