Because these market (line) odds do not meet fundamental betting criterion FAIR odds < LINE odds. Such criterion is required for bettor to have a profit in the long run. It is subject to the theorem which is available on the the website
http://www.newhavensoft.net and in the manual topic
Stake Wizard Guide > Math background. Manuals are available on the download page
http://www.newhavensoft.net/dload.htm.
Cells color coding:
1. Violet (or blue on LCD screens) - events which are NOT recommended to bet on due to unfavourable trends.
2. Green - bet events suitable for betting and meeting the criterion Fair odds < Line odds. The greater the difference, the deeper the green color. Dark green (Fair << Line) requires additional checking from another sources.
3. Red on the correct scores view - indicates the probability of some certain score to happen. The greater the probaility, the deeper the red color.
4. Gray - same as 2 but with unfavorable upward market odds trend.
1 and 4 are the elements of Odds > Antibet filter.
2 and 4 appear when you choose Odds > Market line odds