I intend to have my anti-reflection coated window at roughly 20° angle of incidence the question is if it red shifts or blue shifts in terms of transmitted wavelength
If you tilt a transmissive window with anti-reflection coating and consider transmission in forward direction, then you will get shifts that differ for a classical quarter wavelentth plate (thicker window upon tilt will mean destructive interference in reflection will move to longer wavelength, so transmission to shorter wavelength) and a microstructured coating (surface periodicity will appear shortened upon tilt so reflection minimum will move to shorter wavelength).