One way would be to use ANSI escape sequences.
http://www.pcxt-micro.com/ansi.html

I'm not quite sure what the best way to actually implement these in DOSBox is, but I've attached a .bat file that uses the example from that website which I made using a hex editor. Rename the file to test.bat and run it in DOSBox, and you'll have white-on-black text.

I'm not sure how to make the full screen go white, though. Maybe some ASM code to change the video mode?

You can run the attached TSR from [autoexec]. The parameters for black text on white would be

1CHGCOLOR 0:63 7:0

Apparently just working in MsDos using an int10h function10h will allow me to set the bkg palette color to whatever, but only in regular video modes(640x480) and not in int10h function4f Vesa modes (1280x1024). And so the problem is not with DosBox running in Windows XP but with my program itself. I need the higher resolution with background colors.
Thanks for your replies.