There is much good stuff about Hanoverians in 1815 on the following blog: apologies if you already know of this and have read it....

http://generalpicton.blogspot.com/2018/06/hanoverian-battalions-at-waterloo.html

Amongst other things the author accepts that the Feldjager may have worn green caps, but also suggests that while red caps may have been issued (effectively, as forage caps) to other units, their wear on parade and in battle may have been less-than-commonplace. A shako would be the regulation uniform when facing the enemy.

We will never know, and so artistic licence prevails. And I am quite happy with that...after all, nearly all my Napoleonic figures are much more uniform, and much less grubby, than they would really have been in the field.

👍

Since the Elector of Hanover became King George I in 1714, Hanover and Britain had a close relationship. However, despite the fact that this relationship had some influence on British foreign policy and that the two countries shared a ruler, the states and their armies had remained independent.
Source: https://www.thenewsinsides.com