Help: Command to search & replace word in file?

["Jonathan Gift" <jgift@wanadoo.fr>]

Hi,

Is there a way from the console to use a grep like command to search a file
for, say the word "blue" and replace it with "red"? The key is console and
not within Vim, etc. I'm trying to set up a way to automate changing
.Xdefault setups with a simple Bash script.

Thanks,

Jonathan