My now toddler had constant colds from when she was 18 days old till when she was 5 months more or less

I agree with pps about the elevated head sleeping position, nasal spray and syringe (although here in Europe we're recommended the ones you suck on far more than the bulb syringes - easier to control, they say).

I found that relieving the congestion enough (various sessions of saline and syringe...) so that LO could start breastfeeding meant that then they started to feed, thus swallowing lots and relieving a lot more congestion. HTH!