I'm trying to force vim to use
set spell spelllang=en_gb
If you are just testing those words as the only word in the document, neither is correct: they will both be marked as
SpellCap (word not capitalized). If you put in
Color, or if you put another word before them,
Colour will not be marked, and
Color will be marked as
SpellLocal (wrong spelling for selected region). You can see this for yourself using these:
hi SpellBad ctermfg=red hi SpellCap ctermfg=blue hi SpellRare ctermfg=green hi SpellLocal ctermfg=yellow
By default, on many colourschemes, all of those just render the same way (underline), so you can't distinguish them.
You might also find that out by using
z= (suggest word) on
colour - you'd get
Colour as a top correction.