There are 3 planes involved in a color image. Each plane having values 0-255. By extracting any single plane you can convert the image to a grey scale one.
Try extracting the luminance plane (in color plane extraction).
Use vision assistant initially to get an idea.....
If the crack in really nicely different from the object then applying a proper threshold and removing small particles and going with a particle analysis will be enough.
But we need details.If you can post some images then I can suggest you a method
What muks suggested is a very practical approach. I personally implemented it with object filtering to detect cracks in an online ceramic plates inspection.
to convert a color image to gray scale you can average all three planes or you use different weights for the color planes. Typical weights are
30% red + 59% green + 11% blue