New to image processing, in FMX I'm trying to convert from color to mono.

New to image processing, in FMX I'm trying to convert from color to mono. I can't seem to get the code below to work. Am I doing something obviously wrong?

procedure ConvertAlphaColorToMono( var aColor: TAlphaColor);
var
Mono: Cardinal;
begin
Mono := Min(255,Trunc( (TAlphaColorRec(aColor).R * 0.21) +
(TAlphaColorRec(aColor).G * 0.72) +
(TAlphaColorRec(aColor).B * 0.07)));
TAlphaColorRec(aColor).R := Mono;
TAlphaColorRec(aColor).G := Mono;
TAlphaColorRec(aColor).B := Mono;
end;

Note there are other numeric factors than the ones I'm using above. Good quick discussion here: https://www.johndcook.com/blog/2009/08/24/algorithms-convert-color-grayscale/