The reason would be that for the background() function the alpha channel of your colors is being ignored, for fill() however it is not.

The problem you're having is in this line:

1. palette[p] = color(unhex(strings[p]));

Calling the color() function with a single value usually is used to create a greyscale colors. To also support already packed (A)RGB values, the function is also doing an internal check for the presence of an alpha channel. If it's present (i.e. alpha > 0) then the color() function initializes an ARGB col, if not it interprets the value as greyscale.

You colors are all just 6 hex digits long (with an implied max alpha value), so running them through the unhex() function, will turn them into RGB values, but leave their alpha value at 0. You can either fix this by:

1) adding the alpha channel to your hex values:

e.g. 886644 becomes FF886644 (the FF prefix is the alpha byte)

or:

2) you change the color parsing line to force solid colors:

1. palette[p] = color(unhex(strings[p]) | 0xff000000);

Hth!