What do the different colored bars mean in my CO log?

The different colors indicate the different CO level ranges. The typical range for non-smokers is green (0-6 ppm) while the range for smokers is red (10ppm and above). These ranges are based on a collection of studies using measurements at multiple times of the day. 

Most samples submitted by non-smokers are in the range of 0-6 ppm. Most samples taken from a smoker are 10 ppm and above. Orange (7-9 ppm) represents a borderline range that may be found in some smokers and some non-smokers. This will depend on the time of day, amount of smoking (for smokers) environmental exposure and other factors. 

Your breath sample levels are based on taking multiple measurements throughout the day. This shows how your CO levels rise and fall according to your smoking behavior. One measurement alone may not be as informative for you as you having visibility to multiple measurements over time.