How to filter an RGB image and transform into an BW one

edit

I have been trying to do that:

http://prntscr.com/umv66 -> http://goo.gl/Bgmv2 (You can ignore the red lines, I just added them to illustrate the final objective)

So, what I've done in the first case was: Using MatLab, I transformed rgb -> hsv and picked up the s channel (a.k.a saturation). Then I used Otsu (called graythresh in matlab) to binarize and transform into a black and white picture. Everything worked perfectly.

Now, in the second case this doesn't work so well:

Base img(http://goo.gl/OtK4G) -> S Channel(http://goo.gl/oo8h6) ->Otsu(http://goo.gl/cnfxU)

As you can see, the Otsu filter becomes useless in this case. Is there any workaround or other way I should be taking? Thanks in advance!