How to change every black pixel to a special color?
You can use the LUT or GEQ filters for this.

e.g. with LUT 

ffmpeg -i myVideo.avi
       -vf
       "yadif,format=rgb24,
        lutrgb=r='if(eq(val,0),3,val)':g='if(eq(val,0),3,val)':b='if(eq(val,0),3,val)'"
image%d.png