On my EX1, B is set to ATW (which I seldom use)
A is what I manually white balance with a card,
and Preset is what I dial in from the picture profile.

As I understand, A performs the green/magenta correction, Preset does not.

When I go outside at dusk, this time of year in Denver, and white balance on a card, it comes up as 20,000 kelvin, which seems slightly yellow.

If I set the Preset from the picture profile I can't go above 10,000k and it looks magenta.

You can use color matrix in presets to fix green/magenta

You can use color matrix in presets to fix green/magenta

Thanks, I could do that in post as well. Any thoughts on the "White Shading" inside the maintenance menu?

White Shading corrects for hot spots or uneven exposure and require very careful setup. It has no effect on white balance.

Thanks. I'm leaning toward not making the adjustment on the color matrix Tuomas mentioned because I think it's correct where it stands under normal lighting conditions. Could the magenta push on the preset setting be happening because I can't manually dial the temperature above 10,000k? Is 20,000k under the white card balance unusual for evening shadows? Since I can't go above 10,000k dialing the temperature manually, could doing an offset in the picture profile for a white card balance be helpful? Maybe using an offset for the manual temperature setting could compensate for the magenta. I didn't think of that, but the confusion arises because the scaling of the offset is +/-99 arbitrary units whereas the temperature setting is absolute degrees kelvin, so trying to make this compensation from the flip out LCD is not likely to be satisfactory.