RenderSettings.ambientLight

Description

Flat ambient lighting color.

Flat ambient lighting mode uses color. It has the same value as ambientSkyColor.

using UnityEngine;

public class Example : MonoBehaviour
{
    void  Start()
    {
        // Make the ambient lighting red
        RenderSettings.ambientLight = Color.red;
    }
}

Additional resources: ambientMode, The Lighting window.