At what altitude was the moon at the time?
The atmosphere acts as a prism and deviates red and blue light differently at different altitudes. The effect is most apparent where the air is thickest — at low altitude.
https://en.wikipedia.org/wiki/Atmospheric_refraction covers the phenomenon but only slightly and could be greatly improved in my opinion. Searching on “atmospheric dispersion corrector” will produce more information and a fair description appears at https://astronomytechnologytoday.com/2017/07/06/atmospheric-dispersion-corrector/ but the available information is far from satisfactory in my opinion.
Paul