no its a palette index (position) and a weird one at that (not first or last) if i remember rightly
its been a while (sigh, come on memory - whirr, whirr, click, click whirr )
to test make a bmp with ALL palette positions the test color (i remember i used the xp/win2k 'Background' blue the first time)
insert and test it you'll see the tail off has changed
copy this bmp and change one index at a time (1 to 16) to a seperate new color - green or something (but not black) changing the previous one back to 'blue' as you go.
insert and test
repeat.
when the tail off goes green (or 'something') you've found the palette index for the tail off (which you would use for the backgound of the text also in most design cases)
i would be more help (i have done bmps at home i could just look at in psp) but i'm away from home currently so don't have access to my own computer.
anyway its only 16 colors to test

= a good rainy afternoon task

ps. if you try this out post your findings for other browsers.
pss. ensure the new BMP is the correct BMP format (my memorys twitching that it's ment to be RLE but i could be wrong - sleepy)
psss. i find PSP best for windows palette work btw.
pssss. i will try to remember to edit this page with corrrect info (if its not extended) when i get home (it'll be a while though)