Perlin noise map color transition with uniform thickness

Question

I want to create a evenly sized zone at a specific height to transition between colors. The issue i am having is that i cannot get this zone to stay even in size. Is there a way to achieve this?

I have tried

        if (noise >= .21f& noise < .213f)
                {

                    pixelColour = Color.Lerp(Color.blue, Color.yellow, Mathf.InverseLerp(.21f, .213f, noise));

                }