I am an 18 year professional carpenter and roofer. The use of "shake oil", as we call it here in the PNW is quite common and when used correctly and responsibly can give decent results, though there are much better options out there. As the link says: it does not hide anything at all period. It is just an oil not a stain. It will darken the wood by a shade or so (just like anyother oil on raw wood) but wont hide any discoloration or restore anything to "like new" condition. If you want to restore Cedar shake you first have to pressure wash with what we call a soft tip. This nozzle delivers only 200lbs of pressure +/- depending on the machine, instead of a few thousand. These do not come standard with a pressure washer, you have to order them...anyway you have to remove all patina from the wood (this will NOT remove rust marks). If you have washer marks you didnt do it thoroughly enough; spray till theres no marks. You want to wait untill the wood LOOKS dry to apply the oil. I reccomend a power sprayer (WEAR A RESPIRATOR!) and/or lots of rags. Brushes arent much help; the oil is too thin.
This will bring the shake to an almost redwood hue. The whole point of the oil of course is to postpone decay. As the weather cycles, natural wood oils are lost and the shakes lose weather resistance. For astetic purposes its best to do it right when its installed and every 5-10 yrs thereafter. If being used after the fact the customer should be made aware that not only will the oil NOT hide anything, it will enhance it! its very important that you pressure wash if you want to get anywhere close to "like new" condition. Also because of the nature of wood, different shakes from different parts of the tree(s) will color differently but almost always looks good provided proper and meticulous preperation takes place.
Never ever ever use bleach to clean wood! there are several Hydrogen peroxide based cleaners out there specifically designed for this that will not comprimise the wood and is basically enviromentally safe (kills non vascular plants like moss which is why we use it to demoss roofs) and they work superbly.