@ambra @CeeCee There are endless variations on amba, some are more like a pickle, others like chutney, some a spice centric sauce. For sabich, you want a saucy version, in which the mango plays a background role.
The unripe mango adds an acidic fruity flavor, not a sweet one. Fresh mango will likely get you an overly sweet result, so if you include it, only use a small amount for some fruit flavor. If you can't get unripe mango, you can use amchor (dried mango powder) for the acidity, if this is also not an option, use lemon. Actually, you should add some lemon anyway.
The bright color is from turmeric, which is important. But most important is fenugreek which should be the main flavor component for a sauce made for sabich. You can also add many other spices - chili, paprika, black pepper, cumin.
Also important, is that you must let it stand after mixing - the fenugreek powder is a natural thickener and takes ~3 hours to do so.
Bottom line, in sabich, the amba doesn't have to be authentic, just make sure that its bold on fenugreek and turmeric, acidic, salty and fruity, with just slight sweetness.