I think you have applied im2bw() function on YOUR image. Try segmenting the image. it might give good results.
Other options will be converting image into other color spaces like HSV , LAB etc. If you are taking an HSV image, red and green pixel hue values will be different from blue, so keep some threshold or range to get those pixels.you can also remove light colored image parts using saturation.
use rgb2hsv() for conversion.
pic=imread('image.jpg');
hsv=rgb2hsv(pic);
bw=find(hsv<0.2 || hsv>0.8);
i haven't run the code .. but it might work...
