In a word, very! Most Harry Potter fans feel a strong connection to one of the 4 houses, and wouldn't want to receive merch for the wrong one. The 4 houses are Gryffindor (red and gold), Slytherin (green and silver), Hufflepuff (yellow and black), and Ravenclaw (blue and bronze). If you're unsure what house someone is in, just ask them, and they'd be happy to tell you.