Vertex gradient shader that interpolates between 2 colours based on every vertex Y position.

Vertex-GradientTransparent.shader

Shader "Vertex/Gradient" {
Properties {
	_TopColor("Top Color", Color) = (1,1,1,1)
	_BottomColor ("Bottom Color", Color) = (1,1,1,1)
	_Scale ("Scale", Float) = 1
	_Offset ("Offset", Float) = 1
}
 
SubShader {
	Tags {"Queue"="Transparent" "RenderType"="Transparent" "LightMode"="ForwardBase"}
    LOD 100
 
	Lighting Off
    ZWrite On
    Blend SrcAlpha OneMinusSrcAlpha
	Cull Off
 
    Pass {
        CGPROGRAM
			#pragma vertex vert  
			#pragma fragment frag
			#include "UnityCG.cginc"

			fixed4 _TopColor;
			fixed4 _BottomColor;
			fixed  _Scale;
			fixed _Offset;

			struct v2f {
				float4 pos : SV_POSITION;
				fixed4 col : COLOR;
			};

			v2f vert (appdata_full v)
			{
				v2f o;
				o.pos = UnityObjectToClipPos (v.vertex);
				o.col = lerp(_BottomColor,_TopColor, (v.vertex.y * _Scale) + _Offset);
				return o;
			}
			
			float4 frag (v2f i) : COLOR {
				float4 c = i.col;
				return c;
			}
			ENDCG
        }
    }
}