(Unity3D) Attach to a Camera and on runtime, the script creates a background gradient plane and initially fills it with the colors set in the Editor. Other scripts can then change the gradient color by calling: BackgroundPlane.TopLeftColor = new Color(...); // or TopRightColor, BtmLeftColor, BtmRightColor

BackgroundPlane.cs

using UnityEngine;
using System;
using System.Collections;

public class BackgroundPlane : MonoBehaviour {
  
	static BackgroundPlane _this;
	
	public Color topLeftColor, topRightColor, btmRightColor, btmLeftColor;

	Mesh thisMesh; 
	Camera thisCam;
	GameObject backgroundPlane;
	
	public static Color TopLeftColor {
		get { return _this.topLeftColor; }
		set { _this.topLeftColor = value; _this.setMeshColors(); }
	}
	
	public static Color TopRightColor {
		get { return _this.topRightColor; }
		set { _this.topRightColor = value; _this.setMeshColors(); }
	}

	public static Color BtmLeftColor {
		get { return _this.btmLeftColor; }
		set { _this.btmLeftColor = value; _this.setMeshColors(); }
	}

	public Color BtmRightColor {
		get { return _this.btmRightColor; }
		set { _this.btmRightColor = value; _this.setMeshColors(); }
	}	
	
	void Awake() {
		_this = this;	
		thisCam = _this.camera;		
	}
	
	void Start () {
		if (thisCam == null) {
			throw new MemberAccessException("BackgroundPlane must be attached to a Camera object.");	
		}
		
		float farClip = thisCam.farClipPlane - 0.01f;
		
		Vector3 
			topLeftPosition  = thisCam.ViewportToWorldPoint(new Vector3(0, 1, farClip)),
			topRightPosition = thisCam.ViewportToWorldPoint(new Vector3(1, 1, farClip)),
			btmLeftPosition  = thisCam.ViewportToWorldPoint(new Vector3(0, 0, farClip)),
			btmRightPosition = thisCam.ViewportToWorldPoint(new Vector3(1, 0, farClip));
		
		Vector3[] verts = new Vector3[] {
			topLeftPosition, topRightPosition, btmLeftPosition, btmRightPosition
		};
		
		int[] tris = new int[] {
			0, 1, 2, 2, 1, 3
		};
		
		thisMesh = new Mesh();
		thisMesh.vertices = verts;
		thisMesh.triangles = tris;

		backgroundPlane = new GameObject("_backgroundPlane");
		backgroundPlane.transform.parent = transform;
		backgroundPlane.AddComponent<MeshFilter>().mesh = thisMesh;
		backgroundPlane.AddComponent<MeshRenderer>().material = new Material(
	        "Shader \"BackgroundPlaneShader\" {" +
		        "SubShader {" +
		        "    Pass {" +
				"			ColorMaterial AmbientAndDiffuse "+
		        "    }" +
		        "}" +
	        "}"
		);

		setMeshColors(); 
	}
	
	void setMeshColors() {
		thisMesh.colors = new Color[] {
			topLeftColor, topRightColor, btmLeftColor, btmRightColor	
		};
	}
	
}