It starts from what you put in the cx, cy parameters (c = centre, so cx = Centre X, cy = Centre Y). 
EDIT: Also, changing corners won't make the colour appear, as it still has to search the entire client area nonetheless.
But, for learning purposes (if you are unsure of how to approach the solution), you can do something like the following:
Simba Code:
var
I, Colour, Tolerance: Integer;
centrePoints, Points: TPointArray;
begin
Colour := 16777215; // The colour to look for.
Tolerance := 16; // The tolerance you are allowing for.
centrePoints := [Point(0, 0), Point(MSX2, 0), Point(0, MSY2), Point(MSX2, MSY2)]; // The corners of the client area. If you are using it for RuneScape, then you can just leave it the way it is :)
// Go and find the colour and each of the four corners, and when found, break out of the loop.
for I := 0 to High(centrePoints) do
if(FindColorsSpiralTolerance(centrePoints[I].X, centrePoints[I].Y, Colour, Points, MSX1, MSY1, MSX2, MSY2, Tolerance)) then
Break;
// Check if the colour was actually found or not.
if(I = High(centrePoints) + 1) then
begin
Writeln('Could not find the colour in any of the four corners! :-(');
Exit;
end;
// ... Do other stuff here.
end;

Please note, I wrote that inside of my browser and off the top of my head, so it may/may not compile and require a few additional changes/fixes.