I received this email yesterday:

Recently you bought a Carbtune or accessories from us.

Now we can also supply the Colortune. This is a kit that contains a see-through spark plug that enables you to see the colour of the flame in the cylinder. By adjusting the mixture screw on individual carburettors you can set the flame colour so the mixture is correct. £35 total delivered price anywhere in the world by airmail.
See it on www.carbtune.com/colortune.html

I'm a bit lost as to how valuable or effective this tool might be, but I thought I'd post up and share the news.