No, there's no covering anything up involved. The testing lab tests the hair follicles themselves, which is where the cannabis is detected. You could dye your hair purple, shampoo it 220 times, or shave it all off and require them to go after underarm or pubic hair (in which case they usually just write you off as a failure anyway). It's going to show up. Hair testing is very dependable and accurate and very hard to adulterate, fake or cover up. That's why they use it, because of its accuracy and its long window of detection.