It works as expected for me, using vim-8.2.1690.
- with ":set spellllang=en_gb spell", vim highlights
the word "color" as misspelled but not the word "colour".
- with ":set spelllang=en_us spell", vim highlights
the word "colour" as misspelled but not the word "color".
It uses the color of highlight group SpellLocal.
Maybe your color scheme does not highlight the
the group SpellLocal? For spell errors, Vim uses either
SpellBad, SpellCap, SpellLocal or SpellRare. See
their meaning in :help highlight-groups.
What happens with a standard colorscheme of vim
such as ":colorscheme desert"?
Try also the command :spellinfo. It may show that you're
using a non-standard spell file in your ~/.vim/... directory.
Regards
Dominique