Why is there no color shift on the photo of the M87 black hole?
The picture isn't a &quot;colour&quot; picture - it is monochrome. i.e. It is obtained at a single microwave wavelength of 1.3 mm, and so not at any wavelength you could see (Akiyama et al. 2019).
There isn't therefore any spectral information that would reveal the expected Doppler effect. Any difference of colour in the &quot;false-colour picture&quot; is purely related to the intensity of the emission, not its wavelength.
If one were able to obtain coverage at multiple wavelengths then you might expect a &quot;bluer&quot; (i.e. shorter average wavelength) colour to be associated with the brightest regions.