Changing default OpenGL color interpolation

Is there a way to redefine how OpenGL interpolates color between two vertices? For example, if I draw a line between blue and red vertices, I am looking for the line to interpolate through the color spectrum. (interpolate may not be the correct word at this point). My current solution is to load a texture of the color spectrum and map to it. Is there another way to achieve this effect without using textures, writing my own shaders, or introducing more vertices to interpolate through? (Basically just using a clever combination of OpenGL functionality)

No, you can’t.

Basically just using a clever combination of OpenGL functionality

Those things you cite are a clever use of OpenGL functionality.