You should run the script as 'superuser', just add 'sudo' in front of the command and type your password when prompted.
So try:
sudo /dvtcolorconvert.rb ~/Themes/ObsidianCode.xccolortheme
If this doesn't work, try adapting the permissions:
sudo chmod 755 /dvtcolorconvert.rb
sudo chmod 755 ~/Themes/ObsidianCode.xccolortheme