It is all about testing
Making your own glazes is a steep learning curve. I finally made a wonderful blue and green matte glaze from an adjusted recipe of Katz Burke.
For the basic recipe: https://glazy.org/recipes/27838. You can then add any colorant.
You only need a very small amount of cobalt to get these vibrant colors. The two on the right are from commercial stains (Silex) called Sea Blue.
Here are some tests withcopper oxide and copper carbonate. It even holds it’s colour on dark clay body.
The results on a small bowl
