Exterior colour is exposed to UV
Outdoor products are continually exposed to sunlight and weather, so it is sensible to expect some degree of normal ageing rather than assuming any exterior finish will remain visually identical forever.
Different finishes read differently
Dark, grey and timber-influenced finishes can respond visually differently to light, shade, moisture and surrounding materials.
Use physical samples
Photography and screens are not reliable enough for a final colour decision. A physical sample gives a much better indication of the actual finish.
Consider the whole garden
Paving, brick, render, planting and orientation influence how a fence colour appears. Judge the finish in the context of the proposed garden.
Follow the care guidance
Appropriate cleaning helps keep the surface looking its best; avoid assuming that aggressive chemicals or unsuitable cleaning methods are appropriate.