The whole point of a static node is that you set it in the material instance, not as a material parameter. This optimizes the shader count and shader memory usage.
If you need to change the value at runtime, then you should use a regular parameter, not a static parameter.
Using lerp is pretty easy. Put in two colors (or values, or vectors) and a scalar between 0 and 1. At 0, you get A. At 1 you get B. At 0.5, you get the mid point.
Btw: That parameter at the bottom is a “scalar parameter.” Each color is a “constant vector3.”
To get more insight into materials, download some free content packs from the Marketplace and pick apart the materials that they come with.