I've been learning Japanese for a little while now and I'm still baffled by when to use katakana. All descriptions tell me it's used for foreign words, names, and words borrowed from other languages. But how am I meant to tell what the foreign words are? For example, banana uses katakana, but I could easily write it in hiragana. Am I meant to look up where the word banana came from just to know which way to write it? Same with other words. Guree (gray), pinku (pink) and orenji (orange) all are written with katakana in my textbook. How am I meant to tell that those three are foreign words, but not aka (red) or ao (blue)? I've even seen whole sentences in katakana, or words (though I can't remember which) that seem to be comprised of all both hiragana and katakana in the same word, switching between the two. How? Why? Please someone answer this for me, how to know when to use katakana. Because I can't recognise which words are considered 'foreign' words or words not native to Japan. It's the only thing I can't get the hang of. Thank you!