'Colourful Language' with regards to swearing
Why is expletive laden, or coarse language often referred to as being colourful/colorful?
Oxford Dictionaries define it,
colourful
2.2 (of language) vulgar or rude.
‘colorful words usually impolite in public meetings’
‘And there's some very colorful language in some of those opinions.’
‘This version has had some of the more colourful language removed.’
Nigel J's answer covers most of the relevant points regarding this use of the word "colorful." I would only add that the definition in the OED that refers to this use is tucked into a larger definition referring to writing that is expressive and vivid.
fig. Of language or literary style: highly expressive, vivid; (euphem.) of the nature of or characterized by coarse slang or invective.
This use is attested in 1876.
She did what so many ‘fervent souls’ do—scribbled her heart out in a colorful, barbarous rhyme.
- 1876 - Mary Murdoch Mason · Mae Madden
So in the case of "colorful" being used euphemistically to refer to the obscene, it is the same kind of euphemism we would have if we said "That's some very expressive language," or "that's some very vivid language."
And it's true that vulgarity and profanity carry a message that is highly expressive and vivid; language that is "colorful" in this sense is marked by the strong feelings of whoever composed it.
The earliest reference I can find is in the OED reference to 'colourful' :
1914 in Hist. N. Otago from 1853 (1978) 117 Shouting colourful epithets at them, and waving a Maori kit which contained her supper.
That quote would appear to bear the meaning of strong language, a 'colourful epithet'.
The NGram for 'colourful language' begins just after that 1914 quote, in about 1918.
'Colourful' bears a general meaning :
b. euphem. Disreputable; questionable; notorious.
so I assume that is the derivation of its application to language.
And, again, that traces back to the First World war period :
1921 K. Tappert Viewpoints in Biogr. 55 In spite of Nell Gwyn's rather colorful life, the English people love her memory.
'Colourful epithet' is BrE and 'colorful life' is AmE and all this appears to begin in the 1914/1920 period so maybe it is an expression of those times of American/British intermingling and postwar relaxation of society.
'Colourful' does not condemn the language being used, as Victorian ideals would have done so. It is a comment, not a criticism. I think it denotes the changes in society occurring during and after the First World War and involving the unprecedented social upheavals which happened at the time.