hi, the first squad i painted. i had a hard time with yellowing the armour since i had underpainted black. had to add several layers but the undercoat kept shining through.
Whenefer doing yelloy, you should ALWAYS re-base the area white, even if it's already primered in white. Yellow is such a weak pigment that 1) you need a strong white undercoat to get it to paint smoothly, and 2) everything bleeds through.