I use:
White = 0.5mm
Blue = 0.75mm
Red = 1.0mm
Black = 1.5mm
Grey = 2.5mm
Orange = 4mm
And a pair of crimpers like:
View attachment 188856
Although not those, the blue handled ones I have probably cost about £13 but do the same job.
But the above are just what my employer "got in" rather than any specific standard I think. (EDIT: pipped atthe post - it seems we are using the French colours - why do we not have our own?)
It adds more time to the job but does make for a nicer connection on many occasions, on some its not so good. (like in Omron relay bases.)