The documentation for `notmuch-search-line-faces' says:

> The attributes defined for matching tags are merged, with later
> attributes overriding earlier.

A comment in `notmuch-search-color-line', which implements this, says:

> ;; Reverse the list so earlier entries take precedence

Clearly these two are at odds. The implementation of
`notmuch-search-color-line' follows the comment. That is, earlier
entries in the list take precedence.

The original implementation of `notmuch-search-color-line' followed the
documentation of the user-visible variable, but this was changed by
commit 60ebc849 in July 2012.

I'm inclined to go with the documentation and change the implementation,
but this could obviously ruin things for some users until they reorder
their settings.

Thoughts?