What exactly is the role or logic of colon in Lojban? Is it a genuine and exclusive substitution for "cusku", or can it also behave like a sentence separator as does in English?
There is no colon in Lojban. As Adam said, they and the 'A'/'B' are just English-style dialog indicators.
In the online parser jboski, a colon is interpreted as {pi'e}. But I haven't seen the colon used that way recently.
mu'o mi'e komfo,amonan