Nezar, I use the Durgol Swiss Espresso Descaler all the time on three Gaggia machines. It is probably the best product you can use. The color of the solution after descaling will depend somewhat on the make up of your water. As for discoloring some of the machine parts, that will also depend on your water. I wouldn't be concerned about discoloring unless it included pitting or there were an accompanying part failure (valve or pump). I have damaged other machines using lower quality descaling products.
Also, I have used well water on two different properties over 25 years for coffee making. The water we had in the CA mountains was very hard with high magnesium, and now we have high iron and sulfer water in Iowa. For both circumstances the best thing I did to improve my coffee (and, my shaving too) was to install an Easy Water. I don't have any connection with that company, but I do love the product; it really works. If you sign up for their news letter, they offer sales once-in-awhile. Dan