Brief Answer:
Could be dusty eyes.
Detailed Answer:
Hello XXXXXXX
Thanks for the question.
From your description I suspect you are referring to the slightly dusty color of the white part of the eyes, please correct me if I am wrong.
This generally is not something you need to be worried about, this could happen due to exposure to dust and pollution, or it could just be the normal color of your eyes.
If some amount of pigmentation occurs on the sclera (White part of the eye), your eyes will look brown, and this pigmentation can be because of the exposure to pollution and dust or UV
radiation, as I said earlier. You can prevent this by wearing riding glasses while riding a motorcycle.
The eyes can look a bit dirty even if you don't drink enough amounts of water.
Lastly, I have to mention, it may not be brown as you describe, it may be a hint of yellow, in which case it could mean your
liver might be having some trouble.
The best thing to do now is to consult an eye doctor to make sure if it just dirty looking sclera or if there is anything else going on.
For dirty looking sclera, there is no treatment, you can try and avoid exposure to dirt and pollution, and you should wear sunglasses while going out and while riding bike.
Hope this answers your question, please feel free to ask me if you have any doubts.
Wishing you all the best.
Regards.