You can use WIN API function GetDeviceCaps() to calculate the number of colors supported by the current video mode*). This function will return the number of maximum simultaneous colors current video device can handle. The var parameter will be set to the the number of bits per pixel or 0 in case of an error.
function GetColorsCount (var bitsperpixel : integer) : longint;
var
h : hDC;
begin
Result := 0;
bitsperpixel := 0;
try
h := GetDC(0);
bitsperpixel := GetDeviceCaps(h, PLANES) *
GetDeviceCaps(h, BITSPIXEL);
Result := 1 shl bitsperpixel;
finally
ReleaseDC(0, h);
end;
end;
*) Look at the ChangeDisplaySettings routine (in the Win32 API help) to change the mode at runtime