Here are the two "basic descriptions" form Mossmotors.com.
"Gastester. Tune your car for best performance and low emissions. Gas tester measures carbon monoxide in your exhaust to help tune for best performance and economy. Save the cost of multiple trips to the smog shop, and save our environment. New digital version."
"Colortune is a glass topped spark plug which allows you to see the color of combustion in your cylinders. Colortune takes the guesswork out of mixture adjustment. The kit includes a special glass topped spark plug, reflective viewing system for difficult plug locations, cleaning supplies and a wall chart which includes a three step tuning procedure complete with troubleshooting guide."
As you can see both are spouted to allow you to better adjust the air & fuel mixture of your engine. The Gastester actually looks at the carbon monoxide in your exhaust while the Colortune system simply allows you to use a glass spark plug that allows you to view the color of the flame inside the combustion chamber of the head.
The Gastester seems to be the more accurate system at a glance as it actually measures the emissions coming out of your tail pipe. However of all the folks that commented seem to like the Colortune system best. At a glance the Gastester might be the preferable way to go if you car is still required to be smog tested. However some folks I’ve talked to like to use both, with a slight edge toward the Colortune regardless of any testing.
In the end for most of us it might simply be a matter of cost, as the Gastester is almost 4 times as much as the Colortune system. On the Moss web page the Gastester rings out at about $200 while the Colortune is only about $60. However both can be picked up much cheaper through other Moss distributors like LBCC.