They just weren’t showing up at all with the original glaze – not enough contrast.
I put White Celadon on the turtle faces and bodies and retired. It helped but was still too muted.
Then I just went all out and used porcelain paint (dishwasher safe) and added bright orange to the shell and bright green to the body and head – that’s what I wanted. High contrast, bright colors for the star of the piece, colors that pop.
I also made a turtle handle mug with the same issues. Not enough contrast and the colors were too dull. I use the porcelain paint to see what brighter colors would do – I like it!!