You will see opinions, even from espresso machine vendors that blue-green water after a descale is 'normal.' But as everyone here has said, that color is due to copper compounds dissolved in the water and is not such a good thing because it indicates that you are dissolving (i.e., corroding) your copper boiler, pipes, and/or brass fittings.
If your machine has blue-green deposits inside, then that could be a form of scale, including copper carbonate, and in that case a proper descale would give you a blue or cyan tinge to the descale water. The cause of blue-green deposits is complex, and one possible cause would be the machine having been exposed to water with a high chloride content. Ideally you want to eventually eliminate the blue-green deposits, so that any scale in there is the whitish calcium carbonate limescale, and the descaling solution then becomes nearly colorless. If you have no deposits, then you simply should not descale at all.
As hankbates mentioned, vinegar should be better in this case. Calcium citrate (from citric acid and calcium carbonate) may precipitate and settle in the bottom of the boiler where your pressurestat tube is, potentially causing a hidden clog that prevents the pressurestat from working.
toolate wrote:I see a number of old threads on blue water after desclaing La Pavoni Pro pre Milleniium
0ne of them, with the same title as yours -Blue water after descaling...
- is loaded with chemistry background and practical advice from the late Dr. Pavlis. Highly recommended reading on the subject.