Green and yellow poop after starting BioGaia is actually quite common for mixed-fed babies, from what I've heard, it usually settles down once the gut adjusts to the probiotics, and the colour change on its own isn't necessarily a red flag as long as the consistency is normal and your LO seems comfortable. My friend tried BioGaia for her mixed-fed baby around the same age and noticed similar poop changes in the first week, but said it went back to more typical yellow within a couple of weeks once the tummy regulated. The 2-3 day pooping pattern you mentioned is also pretty typical for mixed-fed babies since formula slows things down compared to full breastfed babies, so BioGaia helping things move along, even with a colour shift, sounds like it could be doing its job. That said, if the green is very dark, mucousy, or your LO seems to be in pain, it's always worth running it by your PD just to be sure.