How to compare colors

How do I make a function that compares two colors and return TRUE if the two colors are the same?

There has to be some kind of "tolerance-variabel", for example:

When Tolerance is 0 the function returns true if the two colors are exactly the same
When Tolerance is 10 the function returns true if the two colors are mostly the same
When Tolerance is 50 the function returns true if the two colors are slightly the same
and so on...

The function could be something like:

function SameColor(Color1,Color2:tColor; Tolerance:Word):boolean;
begin
  SameColor:=abs(GetRValue(Color1)-GetRValue(Color2))+
             abs(GetGValue(Color1)-GetGValue(Color2))+
             abs(GetBValue(Color1)-GetBValue(Color2))<=Tolerance;
end;

...but this does not work very well.

How do I make this function in a better way?