Setting a pixel on the screen

Good evening all,

I have been attempting to set a pixel on the screen to white.  Please 
show me what I am doing wrong:

: ohno show black screen ffff 400 400 1024 * + 2* 1e80000 + ! keyboard ;

Only the ffff and the 1e80000 are in hex.  The screen is 1024x768, 
16-bit, VESA 117h.

Thanks,
Jason