To colourize the diff output on the command line, you may use:
diff file-1.txt file-2.txt | colordiff
// Or:
colordiff file-1.txt file-2.txt
Also, to see changed lines in one colour, and unchanged lines in another colour, consider something like:
diff --old-group-format=$'\e[0;31m%<\e[0m' --new-group-format=$'\e[0;31m%>\e[0m' --unchanged-group-format=$'\e[0;32m%=\e[0m' file-1.txt file-2.txt
Gentoo package:
app-misc/colordiff
