Pleyel, on the other hand, continued to use leather on the two-string pianinos and squares, leaving the grey felt for it's Grand Pianos.
The reason may be that, as Sievers mentions, pianos with two strings were too aggressive on the piano-felt because of the space between the strings.
Most of the Erards have a dark grey and light grey felt in the bass with only the light grey layer in the treble.
early 1830's Erard square piano (C.Clarke)
Here we see some examples of piano hammers where the original felt was left under a new replacement felt.
1840's Erard Concert Grand
1840's Erard square piano
1840's Erard grand with gray felt just visible
1842 Erard Grand (C. Clarke)