The "white food" rule is just a rule of thumb, because (in the United States, at least) if something is white, it's PROBABLY more processed. So, white bread is less healthy than brown whole wheat bread; white rice is less healthy than brown rice; white potatoes are very starchy and have a higher carbload than orange sweet potatoes.
A lot of the standard American diet is white/yellow/beige--there's nothing inherently wrong with white foods, but you should eat the rainbow and make sure to eat "whole" foods that haven't had nutrients taken out of them. And jicama is definitely a low-calorie whole food, so enjoy it! :)