I photographed this Horned Grebe at Bear River MBR in Brigham City Utah on May 20, 2021. I initially labeled it as partially leucistic and sent a link to André Konter who wrote a paper:
Aberrant plumages in grebes Podicipedidae His reply which I have his permission to share:
many thanks for your e-mail. I remain indeed interested in aberrant grebes, especially if the observation is accompanied by such excellent photographs as yours. In spite of the quality of your pictures, it remains difficult to define the exact cause for the color aberration. In your case, I tend to say that it is pastel dilution rather than leucism. The reduction of both melanins could be more than 50%. It would thus explain the light grey feathers in the head (normally blackish), the nearly white horns having just a hue of yellow, the brownish back feathers and the near absence of chestnut colors except for hardly noticeable hues. I cannot completely exclude a kind of ino (where the problem lies in an incomplete oxidation of the melanins and not in a reduced quantity that is produced as in dilution). However, being given the extent of paleness, in this case it should be light ino and the bare parts including the eyes should be more visibly affected. My best guess would be pastel dilution and a good deal of sun bleaching.