If you liked 'Red Mars' a lot, and read it with sheer pleasure - then you should definitely go ahead and read 'Green Mars' and 'Blue Mars.'
If however, like me, you found 'Red Mars' to have some very interesting idea and details, and appreciated Kim Stanley Robinson's research into a broad range of fields for his epic dissertation on the possible ramifications of terraforming a planet, but ultimately found the experience of reading the novel akin to studying a somewhat-boring textbook, then you should probably skip these two sequels.
Unless, of course, like me, you have committed yourself to reading all the Hugo and Nebula award winners, in which case you will just have to go ahead and read them.
Basically, 'Blue' and 'Green Mars' are a lot more of the same, but with even more soap-opera-ish drama thrown in. The characters still exist wholly in service to the ideas/concepts of the book (and some get dropped unceremoniously by the wayside after having served their purpose, which makes the narrative structure feel a bit amorphous.)
Honestly, I found these sequels a slog. However, they did win awards, and other people obviously love them...