You don’t sound that skinny!
I’m 5’9″ and been down to 54kgs when fit and strong and didn’t get many colds. I felt the cold a lot and always hate winter, but I rarely got ill.
Over training and being run down and not getting decent sleep is more likely to get you ill than being slim. I got a lot of colds last year but that was more because I didn’t sleep due to a baby waking me every 2 hours :'(
If you feel the sniffles coming on, take a bit extra vit C and Zinc to give yourself a boost in fighting it off. And go to bed early, if you do this just before it properly takes hold you can often shake it off in a day or two. Drinking green tea and eating brocoli also helps.
It’s hard work not getting ill when riding Cyclocross because it’s such a full-on effort in horrible conditions. If you are getting ill lots, perhaps ease up on the mid week training. Remember that racing is the hardest form of training.