How can I replace a pattern only on lines that do or do not contain another pattern?

Question

Say I have a text containing the words red and blue.

How do I replace occurences of the word blue with the word green only in all lines containing the word red?

Likewise how can I replace blue with green in all lines NOT containing the word red?

Answer 1

To replace "blue" with "green" in lines that contain "red":

:g/red/s/blue/green 

To do the replacement in lines that do not contain "red":

:g!/red/s/blue/green