Can't help you, but there is a bunch of info (and expertise) on the Umarex Boys Club forum.
Hi, pretty sure you can get the gen4 (wide frame)in black and silver (nickel). But the gen5 (narrow frame) was only available in the black finish. Could be wrong, but I've never seen a silver gen5 for sale or photos of. There seems to be a few variations in both 4 and 5, in the machining etc. My favorite is the 5 with machined ejector slot and bakalite grips.
Theoben Fenman, Theoben Scirroco 2000, BSA Cadet Major, Webley Premier MK2, ASI Sniper!
Thanks for the replies guys, very helpful....