Why are cartoon coconuts (on trees) brown?

Buy a coconut in the supermarket. What's the color?

The "nut" is most likely brown and hairy. Sure, it's not like they appear when still on the trees, but who knows?

IMO it's all about being recognizable (and also knowledge) to some extent.

In a similar way, if you see Bananas in a cartoon, they're yellow. Tomatoes are red, apples are typically red, etc. even though these things will most likely have a different color or more variation while still being on a plant.

I think you could even go a bit further and compare this a bit to space fights in fiction: Explosions, sounds, pew-pew-pew, etc. yet you shouldn't hear anything. It's about what people know and recognize, it's not about what's realistic.

Edit:

If you want to, you could even consider brown coconuts to be stereotypical coconuts! :) As a different example, what's been the last time you saw some Bavarian in a (non-serious) cartoon (e.g. looney toons)? Chances are high, he had some traditional costume on, at least some lederhosen. I'm from Bavaria, and I've never worn a lederhosen (since being like 3 or 4 years old; didn't have the choice!). Yet as some caricature I'd most likely have one on me - not to forget a Mass of beer in my hands.

Coconuts brown as they mature, so it's not always an inaccurate depiction.

Cartoons rely on a certain visual "shorthand" for many things that fly in the face of common knowledge, but are subconsciously accepted as part of the cartoon world or the larger entertainment or cultural zeitgeist. Most of us are aware that cars can't possibly operate like they do in cartoons, even without ever having seen or been in a car; likewise, while the yellow/green is obscure trivia to some, most of us can generally guess that coconuts are not perfectly spherical, bang-on-head openable bowling balls that produce a xylophonic bonk in the process. Yet we accept these depictions for their simple, distinct characterization. The purpose of such characterization is usually to set up a "gag" that would be diluted by excess realism or veer terribly off-point with footnotes and explication.

Early black and white cartoons without sound effects may have used solid blacks to depict coconuts over other melons or simple round objects. Brown may have been a logistical choice when going to color, with green possibly reserved for melons, etc.