Bolodski, we can't assume what nationality you might be and therefore which system you might have used. Solfege is used in two ways. In some countries the solfege names are used to denote pitches / the piano keys so that for example the key between the two black keys is always Re which is the same as them being called D. This is known as "fixed Do solfege". This is how the pitches ended up being called in France, Russia, Spain. Another kind of solfege was invented for singing, and here Do is the first degree of a scale, Re is the second degree of a scale. This is known as "movable Do" solfege. In this system, in C major the key between two black keys is called Re = D. In the key of G major that same key is called Sol (still = D) because it is the Dominant (5th) note of the scale.
Which kind of Solfege do you mean? The one that gives different note names to the piano keys (fixed Do) or the singing kind (movable Do)?