How can i apply k-means in gray scale images so that it always set forground is black and background is white(if i used 2 clusters)?

When i apply k-means in a gray scale image ,i found that for some pictures the foreground pixels are black,background are white and vice-versa(I use 2-clusters). How can i apply k-means in gray scale images so that it always set forground is black and background is white?