Red or Blue?

Roses are red,

Roses are blue,

Depending on the velocity,

Relative to you.

 

Redshift and blueshift are shift in the observed frequency of visible light due to relative motion of the source and the observer.

 

For redshift, the sources and the observer are relatively moving away from each other, and the Z value is positive.

 

For Blueshift, the source and the observer are moving towards each other, and the Z value is negative.

Done