Inkscape's patterns include "Ermine", and this is the heraldic symbol for ermine in black on a white ground. It is easy to change the colour of the black spots, but is there any way to change the colour of the white background? (I've also created an ermine fill of my own and added it to Patterns, but exactly the same problem applies, as it seems to with the built-in fills as well.)

There are three heraldic variants on the basic black-on-white that I'd like to be able to get: white-on-black, black-on yellow, and yellow-on-black. But all of these depend on being able to change the background. Is it possible, or with two-colour patterns does the background always have to be white? (It is white, not transparent, because if I start with a shield with, say, a blue fill and change the fill to ermine, I duly get the black on white, not black on the blue.)

The pattern does seem to have a transparent background. When you change the original fill, you are substituting the pattern for the blue, not putting the pattern over.

To put the pattern over the blue, I'd duplicate the area e.g. the blue area, and then fill that duplicate with the ermine pattern... then you will see the blue (or whatever color) behind it.

Additional:
You can create patterns with background colors by grouping the foreground with the background, and only then converting this to a pattern. Use a rectangle in the desired background color. You may then get issues with gaps between the tiles, though, so putting the pattern over a colored object, as tylerdurden suggested, is a good way to proceed.

Grateful thanks to both of you. Putting the pattern over the coloured object is doing just what I wanted. I must now have a go at creating a pattern with grouped foreground and background, though it sounds from what you say about the gaps between tiles as though I'll probably stick to pattern over colour.