[Eric_WVGG]

If the game never has more than three nested levels, you could use [square], then {curly}, then <angle> brakest. You could even do a fourth level with (parenthetical) brackets, but that would mean you couldn't use parenthesis in clues.

[esperent]

I think colors, or even shades of grey for accessibility, would be better for non-coders. Maybe even just put the current bracket in bold.

[brookst]

Side note: colors are fine for accessibility, as long as you think in HSL space and vary at least two of those dimensions. As a red/green colorblind person, bright red versus dark green is fine, as is pink versus bright saturated green. No need to go grey.