While I have no problems with indentation based syntax, it's not very conductive to minimization, so it's a no go for JSON's case.
Coloring things is a luxury, and from my understanding not many people understand that fact. When you work at the trenches you see a lot of files on a small viewport and without coloring most of the time. Being able to parse an squiggly file on a monochrome screen just by reading is a big plus for the file format in question.
As technology progresses we tend to forget where we came from and what are the fallbacks are, but we shouldn't. Not everyone is seeing the same file in a 30" 4K/8K screen with wide color gamut and HDR. Sometimes a 80x24 over some management port is all we have.
Sure, color and indentation are optional, but even without those, I don't see that a comma in the above syntax helps much, even on a 80x24 monochrome display. If you want separators, note that the labels which end with a colon are just that, and just as clear as commas. There is a reason why you tried to obfuscate that in your example by placing the colons not with the labels, but the arguments.
You could, there is no fixed syntax for arrays in Practal, so it would depend on which custom syntax becomes popular. But for expressions that are bracketed anyway, it makes sense to have commas (or other separators), because otherwise you write something like
