In a given colour image, click a pixel, select all pixels that are in a colour distance range D=sqrt([r-r']²+[g-g']²+[b-b']²) < Dmax (selectable via trackbar). This gives you similar coloured regions (many connected components). In a result image mark the selected pixels white and the others black. From the white regions mark the largest red in order to see the largest region similar in colour to the pixel you clicked.