This is my first shader ever, so please bear with me if you see something completely off, but firstly I have to ask is what do the : COLOR0 ect do, why are they there. I know the error has something to do with them, but here's the shader. Thank you for any help possible :3

I have a new problem now , nothing appears to anything that's rendered after the apply of either technique, but no errors are displayed.

How I'm using the shader is here and the shader below it

GraphicsDevice.Clear(Color.DarkGray);
spriteBatch.Begin( SpriteSortMode.Immediate,BlendState.AlphaBlend);

textures.ButtonFX.CurrentTechnique = textures.ButtonFX.Techniques["ShimmerTech"];
textures.ButtonFX.CurrentTechnique.Passes[0].Apply();

spriteBatch.Draw(textures.Button, btnPlayPos, Color.White);

textures.ButtonFX.CurrentTechnique = textures.ButtonFX.Techniques["NormalTech"]; 
textures.ButtonFX.CurrentTechnique.Passes[0].Apply();

spriteBatch.Draw(textures.Button, btnUpgradePos, Color.White);
spriteBatch.Draw(textures.Button, btnQuitPos, Color.White);


spriteBatch.End();      

struct VertexShaderInput
{
    float4 Position : POSITION0;
	float4 Color : COLOR0; 
}; 

struct VertexShaderOutput
{
    float4 Position : POSITION0;
	float4 Color : COLOR0;  
};

VertexShaderOutput VertexShaderFunction(VertexShaderInput input)
{
    VertexShaderOutput output; 
	output.Position = input.Position;
	output.Color = input.Color;
    return output;
}

float4 PixelShaderFunction(VertexShaderOutput input) : COLOR0
{
    return input.Color;
}

float4 PixelShaderFunctionHigh(VertexShaderOutput input) : COLOR0
{
	return normalize(input.Color); 
}

technique NormalTech
{
    pass Pass0
    { 
        VertexShader = compile vs_2_0 VertexShaderFunction();
        PixelShader = compile ps_2_0 PixelShaderFunction();
	}  
} 

technique ShimmerTech
{
	pass Pass0
	{
		VertexShader = compile vs_2_0 VertexShaderFunction();
        PixelShader = compile ps_2_0 PixelShaderFunctionHigh();
	}
}

Thank you, that all makes sense.

I have a new problem now , nothing appears to anything that's rendered after the apply of either technique, but no errors are displayed.
I've updated the shader code and the thread.