Hi,
Colored highlighters are indeed allowed on the LSAT:
Bring to the test center three or four sharpened No. 2 or HB wooden (for Canadian centers) pencils—with good erasers. Mechanical pencils are prohibited. All answers must be recorded on your LSAT answer sheet in pencil. Pencils must be used for the writing sample as well. Pencils or sharpeners will not be supplied at the test center. A pencil and a highlighter may be used to underline passages in the test book. Ink or ballpoint pens are not permitted.
While they don't specifically say that colored highlighters are permitted, all highlighters are - by definition - colored. They haven't come up with a black-and-white highlighter, as far as I know
Thanks!