Are you using highly pigmented acrylics?
Or some sort of craft paint kind?
The only modeling paste i have dont behave like that, its winsor and newtons mineral texture gel. A transculent-grey gel with sand in it. I use winsor and newtons galeria acrylics.
Maybe try a less opaque one?
And if you dont want tp paint over it maybe a sheer watered down layer of paint will make it less muddy?
Or if you have glazing medium use that instead of water.
I once had a horrid white texture paste that did make my colours look muddy. Its was a cheap kind i found on supersale in a non art related store.