Clear is perfect, but just to be different I used black and let it ooze out around the edges forming a black ring around the outer edges of the covers. In my opinion I think it looks better and I can easily see if the silicon needs replacing, nice contrast to my Azure Blue car.
I think that I've posted a thread here about this ..... Try a search for covers/ jacking point covers/ votex. Anyway, forget silicon/adhesives, etc. Just drill a small hole through the raised "T" on the back of the cover. Drill another hole through the section of skirt that it concealed when the cover is in place. Now use a thin tie-wrap to loop through those two holes. Adjust the slack so the cover easily hangs down when it's loose. So if the cover comes free it will always be retained and can be replaced. That has happened on my A2 and saved the cover!