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