you can use the desaturation node paired with a scalar parameter in your material. or multiply your base color by a 0-1 value to darken / lighten it. both methods can be parameterized so you can access them in blueprint too.
you can use the desaturation node paired with a scalar parameter in your material. or multiply your base color by a 0-1 value to darken / lighten it. both methods can be parameterized so you can access them in blueprint too.