The Houdini documentation says this about the "Color POP"
https://www.sidefx.com/docs/houdini/composite/export.html [www.sidefx.com]

"Colors particles based on the pixel colors of an image. It has an explicit option to take data from a COP node instead of an image file."

The COP option is exactly what I want to do but the link in the documentation is broken. I have found the "POP Color" node but this does not have an option to point to a COP.

Any suggestions as to how to achieve this?

You'll need to project the points into some sort of image space. Once you've done this and have a texture coordinate, feed that coordinate into a texture call. The texture call can take a path to a composite node where it would normally take a file path.

for example, in the POP Color check "Use VEXpressions" on the color tab and enter

That's perfect, jsmack. Exactly what I'm trying to do... and you built it for me! Thankyou.