A Redshift

In Doppler Effect:

If \(v_r>0\), then Δλ > 0. For a spectral line in the middle of the visible part of the spectrum, a shift to longer wavelength is a shift to the red. The name applies even if in other parts of the spectrum. A positive radial velocity always produces a redshift.

If vr < 0, then Δλ < 0, and we have a blueshift.