install homebrew if you don't already have it.
install the lolcat gem:
gem install lolcat
then
open .bash_profile
edit .bash_profile file in a text editor, add the following:
read -r -d '' VAR <<'EOF'
then insert ascii art or any greeting you want to appear in beautiful rainbow effect, then close with
EOF
echo "$VAR" | lolcat
save your .bash_profile file
then open a new terminal window and boom: