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human
ELI5: Why does urine come out yellow and not green or blue?

When you're hydrated your urine comes out clear but after a long night of drinking it can come out darker yellow. Why not blue after drinking something blue or brown assuming all the colors mixed together?
gpt
Only certain specific foods (such as beets) and some medications change the colour of your urine to a colour other than yellow. When you drink a blue drink for example, most of the time your urine won’t become blue because the molecule that makes the drink blue doesn’t pass through into your urine. Beets contain a pigment called betanin which does pass through to your urine, resulting in pinkish-red urine if you eat enough beets!

(Also certain diseases can cause your urine to become different colours. Go to a doctor if you have any worrisome changes such as blood in your urine or a different colour urine.)

What gives urine its yellow colour is a pigment molecule called urobilin which is a waste product from your body (it comes from dead red blood cells). When you’re well-hydrated, there’s way more water compared to urobilin, so your urine looks pale yellow. When you’re dehydrated, there’s less water and more urobilin, so your urine looks darker yellow. It’s like when you’re making juice from one of those frozen juice concentrates... the more water you add, the paler the juice gets :)

Hope that makes sense :)