So What pipes do we fit outside? The grey which goes white in a couple of years or perhaps the black that does the same in a couple of extra years.Have you heard of UV light? If you haven't, then you'll be one of the many that puts grey electrical cable, underground brown 4" soil pipe and fittings, and white 32mm and 40mm white solvent weld pipes and pipe fittings outside to degrade.
That's why there are qualified tradesmen to do the job right so the customer doesn't botch the job, create a risk, wastes their money on materials and leaving a **** visual job.