al she seemed concerned about was it being yellow in colour (which it is)
Did she say why she was concerned about it ? What does she think may be causing it ?
For what it's worth my son drank aptamil and never had explosive/yellow nappies. I don't think this has anything to do with formula, especially if he didn't use to have this before. Unless you transitioned to bottlefeeding at the same time these issues started ?
Did you change anything ? Are the nappies the right size ? Are the nappies actually loose on him, or the poop so liquid or abundant that it leaks ?
If the nappies are loose despite good weight gain, just go down one size.
If the problem is with the poop being abundant/liquid/radioactive in colour, he may have got some sort of bug. Does he have any other symptoms ? Fever ? Vomiting ? Fussiness, trouble sleeping ? How long has this been going on ?
I'd ask your HV about why she was concerned about the colour and phone your GP if possible. I called my son's doctor because I was worried he hadn't pooped in a couple of days and they reassured me quickly it was normal.
Going 1-2 a day seems on the higher end of average, I wouldn't worry about that.