I have been using Colorsilk Medium Auburn usually for my entire head after I get my hair cut... this time it seemed to come out too light. What should I use to tone it down?

If you want to tone it down, choose a color that's one shade darker.

If it is too light use the next shade down - If it is too gold like you said in your heading - then you need to use a ash colour in the next tone down... hope this helps
Good luck

Wella's UK hair color chart is a little different. Level 6 would be cobsidered light brown, however, Wella's level 6 is dark blonde and their level 5 is light brown. Choose a light ash brown color to neutralize the red tones.