People with brown skin is always dirty.
Why do you think brown skin can't be clean?
Look, I just don't like the look of dark skinned people so I assume them to be dirty. Dunno why.
There are many stereotypes of people of color and them being dirty is one of them and it can be hurtful to think that others think you are dirty because of the skin you were born in. You would be hurt if people thought you were dirty because of the color of your skin.
It's not a bad thing to think people are dirty if they had dark skin.
Don't you think you are making a bit of a broad generalization here though? It's not okay to assume just because someone's skin is darker that they are dirty.