Can boobs just get it wrong?
Regarding the colds thing... I think that it has a lot to do with genetics. Some people just get sick more than others. BFing can boost immune functions, but it's not the end-all-be-all.
As for your previous issues with foremilk/hindmilk imbalance, etc, I don't really know why you've experienced them. It seems to me that some women just come across more problems than others. Just like some babies just cry more than others. I know people who have had mastitis and/or thrush for 3 months straight, people who had absolutely no pain bfing from day 1, people who have babies that cry all day long, and people who have happy all the time babies... doesn't mean that any of them are doing it wrong.
I think that sometimes the bfing community likes to gloss over many of the natural problems mothers have, so that it doesn't turn off new mothers to the idea of bfing.
Our guy (bf exclusively for 6 months, continues to nurse on demand and he is now 19 months) had 8 colds his first year of life. Just is, and the average baby/toddler gets 8 colds a year. I do find that IF I get the cold as well his cold symptoms are milder (b/c he's getting antibodies from me)--the worst couple of colds were ones that I did not get.
So, I do think that bf can lessen cold symptoms and perhaps occasionally prevent colds--but I don't think that your child getting colds mean something in bfeeding is not working.