"Rzecz" can be used in sense of "przedmiot" and actually it is used more often than "przedmiot" but it can be also used in abstract sense. For example the official name of Poland in Polish language is "Rzeczpospolita Polska". Where "Rzeczpospolita" is a literal translation of Latin res publica (Rzecz + pospolita) and means "Republic".
Przedmiot can also be used in somehow abstract sense(cf. przedmiot nauczania) and I'm not so sure about frequency of use between the two.
Also, while official title of Poland in English is indeed Republic of Poland, "rzeczpospolita" on it's own is better translated as commonwealth(republic is "republika" in Polish). That is also how Rzeczpospolita Obojga Narodów is/was usually translated into English(Polish–Lithuanian Commonwealth). ;)
But the origin of "Rzeczpospolita" is Latin res publica. Besides, "Commonwealth" is a historical name. And even if "przedmiot" is used in abstract sense, they are not interchangeable. "Rzecz" is closer to English "thing".
If one of the definitions of przedmiot is "subject" then why was I marked wrong to say colorful subjects. Subjects can be colorful- at least in English.
What is the equivalent of the informal and extremely non-specific expression "colourful stuff"?