The function "bwboundaries" traces the exterior boundaries of objects, as well as boundaries of holes inside these objects. It returns a P x 1 cell array, where P represents the number of objects or holes. Each cell in the cell array contains a Q x 2 matrix where Q represents the number of boundary pixel for the corresponding region. Each row in the matrix contains the row and column coordinates of the boundary pixel. I believe this addresses your first question.
In regard to your query on obtaining the color of the boundary pixel, you can use the original image matrix I and the coordinates of the boundary pixels. If 'r' represents the row coordinate and 'c' represents the column coordinate, the pixel intensity at location (r,c) can be found using I(r,c,:) (if I is not a 2D matrix). In case I is a grayscale or binary image, the pixel intensity is directly computed using I(r,c).
