The fill style. See the Constants table for valid values. The default value is STYLE_SOLID. Color is valid only with STYLE_SOLID.
This still seems to be the case at 3.5 - is that really the case??
We do not have plans to add a "layer list" or "table of contents" widget to manage map layers and sublayers. We've resisted adding this to the API because we don't want to encourage building generic, kitchen sink style viewers. I realize that we (Esri) do not have a consistent story on this as we still provide generic viewers for other platforms/technologies. We're open to discussing it further, but I personally agree with anti-viewer, anti-portal sentiments that some have expressed online for the past couple of years.
- Derek Swingley (ArcGIS API for JavaScript Team Member)
Adding some more details for everyone's reference, since I spent too much time trying to solve the same problem...
I don't have a definitive answer but if you look at the documentation for SVG path drawing (Fills and Strokes - SVG | MDN ), it's actually not technically possible to "fill" with a hatch pattern. This means the symbol would have to manually draw each hatch line as a separate path which would be ugly code and a performance hit. One potential option would be to create a "dynamically colored" image to use as the fill, but I didn't find a simple solution for that either (EDIT: actually that is what's implemented in Simon's solution below, but it uses data:Uri, which aren't fully backward compatible to older browsers).
So I can't really fault the Javascript team for skipping this implementation but a simple note in the docs "if you want a colored hatch fill, (which you probably have in your map service), use a PictureFillSymbol. Here's our black & white ones...." would have saved hours of wasted time.
I agree with other commenters that the non-solid fills in polygons should be able to have different colors.
However, I found that when I define an outline color for the polygon, the inner hatch will have the same color when printed to a PDF (even though the inner hatch shows black on the app).
This might not be the best solution, but at least users can still have different colors on the printed maps.
Hope this helps someone!