If I have time tomorrow I'll be painting my coolant and washer fluid tank. I can clean all I want, but the tanks do not stay clean, and they look bad right now. I won't be doing black though. I probably won't need to leave an area viewable on the washer fluid tank since I can add fluid when the DIC tells me to.
Did you use engine paint for the tank? I like it black actually, it doesn't attract your attention (well it does now since it'* so shiny compared to the rest of the engine...lol). As long as you can see the fluid fine yourself. Now just shine up everything else under the hood.