I’ve had a quick look at TW 5.2.1 prerelease to read about the new filter cascade mechanism.
In the Control Panel > Advanced > Cascades > Tiddler Colour, I noticed that ‘colour’ is used 6 times, whereas ‘color’ is used 3 times. 2 system tiddlers use both words in their names!
I would tend to think that using two different spellings for the same word in a ‘code-writing’ context might be prone to confusion and memory overload from users. My two (euro)cents, as a non-native English speaker. What do you think?