Finding a circle in VB6

Currently I have an array of pixels 640,480 displaying a picture. I am trying to find the diameter of the hole in the ying as shown in the picture. I have a function that can detect different colours based on a threshold.

The way of doing this im currently working on runs through x, notes when the colour turns yellow then another colour again, this works for one orientation, however the picture can be in any orientation.

Any help would be much appreciated

http://tinypic.com/view.php?pic=11v4o4z&s=8#.VSsXO_mUfHU

http://tinypic.com/view.php?pic=m7aszr&s=8#.VSsYPPmUfHU