Sooooo, wavelengths (λ) become longer when something moves away (redshift) and become shorter when something moves towards you (blueshift).

For a red flag (λ0=610nm) to become a green flag (λ1=549nm), it has to move towards you quite fast. But how fast is 'quite fast'?

Using the formula

flag_velocity / speed of light (c) = difference in wavelengths / starting wavelength

we get

flag_velocity = (610-549) / 610 * c = 61 / 610 * c = 1/10 * c

This means: the flag has to move with about c/10 = 30 000 000 m/s = 108 000 000 km/h = 67 108 100 mph. Yeah, that's quite fast.

(Disclaimer:

1. use info on own risk

2. values for λ were chosen in a way to make calculations easy. There is no info on what shade of red or green the flag is. The final result will be about the same.

3. With speeds at around 10% of c, I should use the formula considering the relativistic doppler effect... However, i wont. Thanks.)