I used black, for that professional look. You only need a small pinch for this, so you might want to save this for when you are already using some Sugru for another task. Alternatively, do something creative
with the leftovers!
Clean the lens cap to make sure there's no dust or grease on it. If your cap has a smooth surface you might want to roughen it up slightly with a file, knife or sandpaper to make sure there's a "key" for the Sugru to stick to. Mine had a matt textured finish so I didn't need to do this.
Stick a smallish blob of Sugru onto the front of the cap, about half an inch wide by a quarter of an inch high.