The Alan Lee edition was first published in 1997 with a paperback available in 2000. The product info says this has 22 illustrations.
The 2008 paperback edition claims to have 26 illustrations.
The 2020 boxset edition has 27.

Is the above product info right? Were coloured paintings added over the years?

I know The Silmarillion had various different numbers of Lee illustrations:
2000 - 20 illustrations
2004 - 45 illustrations
2008 paperback - 16 illustrations
2021 - 49 illustrations

I assume the LOTR had the same number of illustrations on every edition until the Folio and 2024 deluxe.