In terminal, how to get "say" to say things "right" ? ie using custom phonetics?
The Speech Synthesis Programming Guide describes the embedded speech commands that you can use to adjust how the speech synthesis engines process your input text.

The [[inpt PHON]] command enables the use of phonemes and phoneme modifiers.

For example, the following text includes a phonemic version of your chosen word  (select the text, Control-click, and select Speech > Start Speaking to hear it directly in Safari):

If you don't like how I say "colossians",
then perhaps you will be satisfied with
"[[inpt PHON]]kUH2lAASUHnz[[inpt TEXT]]".


This syntax is accepted by the say command in the shell and AppleScript (though you will have to modify the (above) use of single- and double-quotes to suit the string literal syntax of your chosen context).