Photopolymer Stamps

Do you mean to use on the stamp, or on the paper once it's been stamped?

If it's to apply colour to the stamp in order to stamp on paper, you typically use an ink pad. There are a ton of brands out there, you can find them at any stamp store.

To colour the image once it has been stamped onto paper, it depends on what look you're going for. You can use coloured pencils, alcohol markers (ie. Copics), or you can even watercolour them using paints or watercolour markers like Tombow.

Here is a video from scrapbook.com about getting started with stamping:

youtu.be/dhLahvFIvO4