When used as an interrogative determiner, some speakers may considerwelcher rather formal and may usewas für instead, e.g.Was für eine Farbe hat dein Kleid? instead ofWelche Farbe hat dein Kleid?
Whenwelcher isn't used as a determiner followed directly by a noun (Welche Farbe ist deine Lieblingsfarbe?) and is directly followed by the verbsein, it is mostly used in neutral formwelches for all genders and plural (Welches ist deine Lieblingsfarbe?). When it agrees with the gender and number, it is usually meant as one from a number of given options (Welche (von diesen) ist deine Lieblingsfarbe?)
Welcher as a relative pronoun is virtually never used in the vernacular. It occurs more often in literary German, but overusing it is a typical trait of a pretentious style (generally corrected by editors). The normal relative pronoun in all registers of German isder and its forms.
Welcher and its forms may be used avoid a doubled word. For example
Das Lied, welches das Kind vortrug, wurde vor langer Zeit geschrieben.
avoidsdas das. Thewelches version would still not be preferred in general though.
German does not distinguish between restrictive and non-restrictive relative clauses and relies solely on context to determine which is meant. (See the usage notes atwhich.) Thusder and its forms are still preferred when English prefers "which":
Paul schenkt sein Auto, das noch neu ist, seiner Tochter.
Paul is giving his car, which is still new, to his daughter.
