I think it depends on the runtime, but usually if it's 0 to 1 float values, colors are multiplied per channel by default and applied as vertex colors. Then the shader program multiplies that color with the colors on the texture. That means a white slot is the normal color, and everything else will only darken it.
Here's a sample of how vertex colors are calculated.: https://github.com/EsotericSoftware/spine-runtimes/blob/master/spine-xna/src/SkeletonMeshRenderer.cs#L116-L126
Again, the way to turn it white probably depends on the runtime.
For example, if the framework/engine lets you, you can push a second vertex color to the mesh and let the shader program handle it in addition to the (animated) colors coming from Spine's system. Engines like Unity doesn't allow us to do this so we have to use other means.
