Actually its a 87 what would cause my windshield wiper reservoir to be brown ?

Thanks its probably a combination of all of the above It really hasn't been driven for 2 years Basically sitting on the street maybe once a month . We are very close to the ocean but I dont know if that would have any effect although its seems to always be windy here or windier then inland Maybe the cap wasn't put on tight at some point. I just started driving it popped the hood & couldn't believe my eyes Looked disgusting & I didnt see a valve to empty it so Ill srcub it out funny thing is its working just fine thanks