applying conditional formatting to numbers in brackets

I'm assuming that you have numeric data formatted as percentage, with parentheses for negative values.

Create a new conditional formatting rule using "Format only cells that contain" and then specify "Cell value less than 0" for the red, and another rule that is "Cell value greater than or equal to 0" formatted green.

(If your data is instead text data, then the rules would be different, but that would also not be a good practice.)