There's normally no need for ${var-} to get default values, and you can put && inside. I'd write that as:

    [[ -z $NO_COLOR && $TERM != dumb ]]

As another comment pointed out, they're erroring on unset values. I don't think the line noise is worth it, but it will catch spelling mistakes, I guess.

No. I'm not sure what the "-" is doing in "${TERM-}", but if $TERM was for example set to "-z stringhere" then that would be interpreted as two arguments to "[[" instead of one, and would ensure that it always returns false, for example