You're sick, you're blowing your nose into a tissue, and then... you look, right? Isn't green mucus supposed to mean something? Actually, your snot can come in a rainbow of colours, with a progression from white to yellow to green as your body fights infection.
Doctors used to say that green mucus indicates a bacterial infection that can respond to antibiotics, but that actually isn't true -- it really just means that your immune system has been fighting something (often a virus) for a couple of days. The green colour comes from a protein in the guts of the courageous white blood cells that sacrifice themselves on the field of battle (your nose).
If you're curious about the other colours, this chart from the Cleveland Clinic gives the rundown on the rainbow: pink or red indicates blood, brown is usually just dirt, and true black mucus can indicate a rare fungal infection.
What the Color of Your Snot Really Means [Cleveland Clinic]