DRAWING

Try something like this,

LONG IF gGWorld& <>_false
   gPixMap& = FN GETGWORLDPIXMAP(gGworld&)
      LONG IF gPixMap& <> _false
         LONG IF FN LOCKPIXELS(gPixMap&)
            CALL GETGWORLD(OldPort&, OldDev&)
            CALL SETGWORLD(gGWorld&, 0)
            BaseAddress& = FN GETPIXBASEADDR (gPixMap&)
            RowBytes& = PEEK WORD ([gPixMap&]+_rowBytes) AND &H3FFF
            PixelAddr& = BaseAddress& + gPixelV% * RowBytes& + (gPixelH% << 2)
            PixValue& = FN COLOR2INDEX(gNewPixRGB)
            POKE LONG PixelAddr&, PixValue&
            CALL SETGWORLD(OldPort&, OldDev&)
            CALL UNLOCKPIXELS (gPixMap&)
          END IF
      END IF
END IF

NOTES!

gPixelV%, is the Vertical position in the window of the pixel you want to change.
gPixelH%, is the Horizontal position in the window of the pixel you want to change.
gNewPixRGB, is the 6 byte variable you are using to store the new settings for the pixel, ie DIM gNewPixRGB.6