Hi. I'm glad you asked this question. The colon sign in mathematics has seven different uses according to what I found on https://mathworld.wolfram.com/Colon.html

Your example, I believe, is item 3: "To give a name to a map, e.g., f:x|->x^2 (which is equivalent to the function notation f(x)=x^2)." In Lesson 4 of An Introduction to Braille Mathematics Using Nemeth Code within UEB Contexts (4.11.3 "Mapping Notation), it says to transcribe this colon as an unspaced symbol. In your example, this means ELI g PI colon x (Letter g is preceded by a space and followed by a punctuation mark, so it needs an ELI. Letter x is followed by a comparison sign so an ELI is not needed for x. A punctuation indicator is required before the colon.)

_% ;G_3X $O ?1/2#X+6 _: