Women come in different shapes, some carry their weight well spread all over or do not carry extra weight, others carry more on the belly and waist area, those are the basic left of the chart.
Some girls though keep most of the extra weight on the hips and upper legs. If you have a standard tights but she is wide there, they would not stretch enough, therefore they do have extra line at the top.
Now think about your girlfriend, look for the row with how tall she is in centimeters.
Find her weight at the bottom, but allow for the diversion of the weight as indicated by the top row.
The wider the hips and upper legs, the bigger the size she needs.
Basically they sell a small, a medium and a large, (or XS/S, M, L/XL) as indicated by the numbers 2 to 4, but they are kind enough to indicate which metric sizes fit in which size categories.
The colours seem randomly chosen, just to help you see the different categories more easily.
As pointed out in a comment, err on the bigger side if unsure.
A bigger size of tights will always fit, if not as nicely, a smaller pair may not fit and are uncomfortable if you manage to squeze yourself in.
The thicker and warmer the tights the more important to buy bigger rather than smaller. Bigger thick tights are warmer than the right size.
