Difference between underside of Den Nobile leaves

[Diane56Victor]

Hope someone can help me with a diagnosis or put my mind at rest about this.
I have a number of Nobile Dens bought from different places and on the underside of the leaves some have a silver look while others look a normal colour.
I've tried wiping to see if any colour was on the cotton pad from squashed bugs....there isn't. The silver leaves are not sticky, but it wont wipe off.
I've checked constantly for bugs there non that I can see and believe me I've looked.
All are growing well. Those with the 'clear' leaves have not changed to look like the 'silver' ones or vice versa.