Blue with a yellow filter?
if my understanding is correct, the "writing" isn't visible until the filter is applied. the filter that makes the characters visible also determines the colors of said characters. adding another color filter (just for color) on top of that could introduce static electricity that could ruin the display or cause malfunctions.
so, although, i'm not the expert on this technology, i'm thinking one polarized filter would likely be what is used to determine display color.
i have one display that didn't show up well in the sun light. i reversed the polarized filter (flipped it over) and instead of the background lighting up blue with black digits, the digits light up blue with a black background.