I am using ConTeXt  ver: 2007.08.29 19:19 MKII  fmt and the pdftex engine

\definetextbackground[A][backgroundcolor=red]
\setuptextbackground
    [backgroundoffset=0pt,
    offset=0pt,
    frame=off,
    location=text,
    background=color,
    backgroundcorner=round]

gives a yellow background 

and 
\definetextbackground[A][backgroundcolor=yellow]

gives a gray background.

Any suggestions about to fix this?

Alan