I will need to paint a very small area of uPVC windowsill as part of a bathroom refurbishment. Would it be possible to buy a sample pot of Rust-Oleum Combi-Colour? (Colour would probably be RAL6018 yellow-green.)

Assuming the Upvc is clean, dry, sound and prepared as detailed in the technical data sheet we can recommend Rust-Oleum CombiColor Multi-Surface.

This is available in many RAL and BS colours in a gloss finish.

Unfortunately the manufacturer does not product sample pots but it is available in 1ltr tins.