The inside but I guess you've answered the question, thank you.Do you mean clean as in clean on the outside or clean on the inside?
The reservoir itself isn't crystal clear plastic. Its a thick, opaque type of plastic and with clean fluid inside would look slightly yellowish. Unless you have dirty PS fluid then the burnt, black fluid would make it look dirty.
Flush the fluid with new fluid and/or simple green and toothbrush the outside.