The principal points of this sketch are the apparent yellow color of the planet, the reduced contrast of features, as well as the reduced effect that planetary filters have on specific features. All of this can probably be attributed to the dust in the atmosphere. The smaller sketch shows how the white southern region was filtered by the Wratten #11 so that only the white Hellas Basin was visible.
This observation was done with my 102mm f/9.8 achromat at ~133x on my driven Polaris mount. In addition to unfiltered, the following filters were used: Wratten #8 light yellow, #11 yellow-green, and #82A light blue.