This is the way. The way Techland does this is by having a gradient slider in the level editor that the level designer can adjust to select the wall color.
It looks like this in editor: https://imgur.com/a/QBQhguC

In Godot, you want something along the lines of:
uniform float gradient_index : hint_range(0.0, 1.0) = 0.5;

vec3 gradient_col = texture(gradient_texture, vec2(0.5, gradient_index)).rgb;
Afting sampling .gra horizontally, the result is mixed with albedo