The visible color of a pattern fill is the FOREGROUND. A plainly-filled yellow cell is a
pattern fill with patternType solid and a yellow foreground; the background is the color
BEHIND the pattern marks (auto on a plain fill) — it is exactly what the CSS renderer
paints for solid too. The fill's generic getColor() returns the background, which is
how a yellow cell once reported pattern #000000 (auto).
Describes a fill —
solid #FFFF00,darkGray pattern #FF0000 on #FFFFFF,gradient.