Much care should be taken when selecting insulators to use on your antenna. If you choose the wrong type, all sorts of problems can be seen, the biggest being IC (Insulator Correction). IC is the result of \'lossy\' materials being used in the plastics used to create the insulator. Most black insulators have carbon within their makeup. Without going into too much detail, this type of insulator will electricaly shorten your elements meaning you antenna will not be on the frequency you need it to be without cutting a new set of (longer) elements!
All G0KSC insulators have been checked and no IC is needed at all. Enjoy building you antenna but most of all, enjoy using it for many years to come.