Beets would be fine too...but harder to discern the colors.
Any cleansing can cause detox reactions if the toxins are being loosened and released from tissue and organs faster than the body can expel them. The Bentonite does do a good job of absorbing them, but as long as they're inside the body they can stir up reactions. Also, make sure that you're well hydrated...it helps to dilute and flush toxins faster.