A couple more things while I'm in idle mode with Landsat under maintaince.
If you are counting pixels for areas of each color, then the graticule (lat,lon lines) covering these pixels should be removed. A cheap way of doing that is to simply bump the count in each bin proportionately so that total pixels add up to the Arctic Basin.
The graticule should never have been flattened onto the map in the first place: the gif89a format as modified in netscape 2.0 took care of this twenty years ago, not that many mapmakers noticed. That is, make the map in gimp as 3 layers, graticle, map, graticule on map via 'new layer from visible'. Gif is limited to 8-bit indexed colors, 256 of them, which can be limiting (amounts to compression) for high resolution color photography.
Then provide the map as an animation that finishes on the combined last layer but does not loop. That way, end users sees the final flattened product but can recover individual map layers simply by saving the image to disk and opening in Preview, ImageJ, Gimp etc.
This is better scientific practice. After all, we're headed into GIS and a map is just one component of that. For instance, there might be dozens of other layers such as ocean depth, surface temperature, ice thickness, wind velocity field, time series and so on.
The idea of GIS is correlations and math right on the image. Note ImageJ provides 19 operations under Process --> Math and Gimp 20 -- and that's before a kazillion free plugins. So you can do everything from divide to boolean logic on pairs of layers. And it all can be dumped into numbers if you just have to work temporarily in Excel stacks or R etc.
In collaborative GIS -- and nowhere is the need for that greater than in climate change -- scientists share layers. Right now we have a lot of idiots flattening down their layers. To where they can't be re-purposed.
In theory you can obliterate an embedded graticule by replacing black pixels with nearest neighbor colored pixels. If you can lift the graticule with the color-picker settings, the temptation in these radial coordinate projections is to replace exactly that selection with what is there after a few degrees of rotation about the north pole. However that computation will produce pixels not in the color key.
Since pixels always form a rectangular array, the alternative respecting that geometry amounts to replacing the graticules with pixels displaced, say to the north and then to the west of the remaining vertical line. This keeps you within the color key. The north pole should be marked in a separate layer to maybe put it back in later.
In practice, this gives so-so results. Often you cannot even lift the graticule because it has been so dithered out making diagonal lat,lon lines and circles. This could have been avoided had the graticule been properly posterized and thresholded to begin with (so it makes dotted lines).
I have not located online graticule vector graphics for our projections that can be rescaled to fit the given map scale. One-size-fits-all raster graticules will need cleanup as described above after rescaling and rotation.
I see quite a few sea ice maps with the North Pole not indicated. If the map is centered (not all are), it will be halfway over and up in terms of pixel coordinates. Otherwise, it has to be located using crossed lines between pairs of well-separated land points or by fitting a land mask with known pole. I am of the opinion the NP should be marked up; the lat,lon are usually a standard integral rotation from Greenwich and it is not too invasive to mark a few crosshairs where meridians and latitude meet.