“Until EOL” is easy: DELIMITERn open = :, close = ((EOL))
Unfortunately, stop-on-other-whitespace is harder; not sure how it could be done natively in a Notepad++ UDL.
There are two alternatives that I can think of:
just define it as a delimiter with ((EOL)) closing it, but set the STYLE for that delimiter to allow nesting of anything else (that you have defined). For example, if = was defined in Operators 2 as magenta, Number defined with blue foreground and cyan background, Keywords 1 defined as Green/Bold with Area as the keyword, and DELIMITER1 defined with open = :, close = ((EOL)) and foreground=red, bg=yellow, nesting=operators2+numbers, I see:
[image: 1579298351581-7f97a779-14f3-4046-b719-3abb641fdf54-image.png]
which might be enough for you.
Otherwise, you can add extra highlighting to a UDL language using regexes via the script EnhanceUDLLexer.py that @Ekopalypse shares in this linked post