Evolving kaiju?

I don't think age or evolution has anything to do with it. I think it's different colored bio-luminescence to separate the look of the kaiju a little more.

I also think whoever is sending the kaiju over pick and chose the toughest kaiju that is even stronger than the previous one after the previous kaiju was killed. Next in line is one that is even stronger and not so easy to kill.

I agree with The Dude the first Kaiju that attacks just has a tough hide, The fourth Kaiju clearly has power strips, data embedded skin. As we learn to kill them better the Kaiju and their orchestrators do the same. All of them have Toxic blood.