The sidebar support that exists in the macros at the moment does support a background colour. It would also be relatively easy to apply a background colour to things like headings which don’t split. We can do this because they are a chunk of text on the page that doesn’t need to be split up, so to put a background colour on the page, it literally works out how big the text block is, paints a square that shape on the page and puts the text on top.
While XeTeX includes the concept of colour in the a font, there is no concept of a background colour in it, thus we needed to take the above approach. I guess it would be possible to typeset a paragraph and then put a box behind every single line, but the problem here is that ascenders and descenders will likely get painted over by subsequent lines or people will complain about the boxes leaving stripes on the page.
Another option is to write magic bits of code that do something clever with the start and end points of a paragraph, but I don’t understand PDF-code well enough to know if it is even possible to have something later in the file painting below what’s come earlier.
That is of course talking about paragraphs, not words. A cheating method might be to draw a box behind every letter, but this would probably have some even uglier side-effects than the lines I mentioned earlier
To implement background colour like you are used to in Word, so that it can apply to everything in a font, beautifully without ugly hacks, someone would probably need to modify XeTeX itself (and device drivers) to do this. Whether that change is a week of programming effort or three months, I’ve no idea.