uses
TypInfo;

{ ... }
var
i: integer;
aFont: TFont;
begin
for i := 0 to aComponent.ComponentCount - 1 do
begin
aFont := TFont(GetOrdProp(aComponent.Components[i], 'Font'));
if assigned(aFont) then
aFont.Color := clWhite;
end;
end;
With D4:

{ ... }
var
i: integer;
aFont: TFont;
pi: PPropInfo;
begin
for i := 0 to aComponent.ComponentCount - 1 do
begin
pi := GetPropInfo(aComponent.Components[i].ClassInfo, 'Font');
if assigned(pi) then
TFont(GetOrdProp(aComponent.Components[i], pi)).Color := clWhite;
end;
end;