Hi Ryn,
Discharge that looks greenish often means infection. However, it doesn't sound like there is a foul odour which wouold be expected with infection also.

If there has been a tiny bit of blood loss that has combined with your usual mucous this can also make the colour a dirty greeny yellow (oh gorgeous hey!). Perhpas this could be what has happened? Maybe you are about to get your period and just the start has tinged the colour. Trichomonas has a green discharge that is usually a bit frothy and smelly. However, it doesn't sound like you have any other symptoms and it was a one off event. It does sound to me that it could be old blood stained mucous...

I would recommend you see a gp and have a swab just to make sure that there is no infection.

Good luck Ryn