Efficiently rendering lots of the same mesh in Unity3D, but with different colors?

MaterialPropertyBlocks are perfect for this situtation.

I followed a quick tutorial, and managed to boost the performance by a lot.

The behaviour below always passes the Color stored in its MaterialColor property to the GPU via a MaterialPropertyBlock, thus letting instanced meshes have different i.e. colors.

public class ColorPropertySetter : MonoBehaviour
{
    //The color of the object
    public Color MaterialColor;

    //The material property block we pass to the GPU
    private MaterialPropertyBlock propertyBlock;

    private Renderer renderer;

    void Start()
    {
        //create propertyblock
        propertyBlock = new MaterialPropertyBlock();
        //Get a renderer component either of the own gameobject or of a child
        renderer = GetComponent<Renderer>();
        //set the color property
        propertyBlock.SetColor("_Color", MaterialColor);
        //apply propertyBlock to renderer
        renderer.SetPropertyBlock(propertyBlock);
    }

    void Update()
    {
        //create propertyblock
        propertyBlock = new MaterialPropertyBlock();
        //set the color property
        propertyBlock.SetColor("_Color", MaterialColor);
        //apply propertyBlock to renderer
        renderer.SetPropertyBlock(propertyBlock);
    }
}

The shader below uses the given MaterialPropertyBlock and also instancing.

Shader "Tutorial/048_Instancing" {
//show values to edit in inspector
Properties{
    [PerRendererData] _Color ("Color", Color) = (0, 0, 0, 1)
}

SubShader{
    //the material is completely non-transparent and is rendered at the same time as the other opaque geometry
    Tags{ "RenderType"="Opaque" "Queue"="Geometry"}

    Pass{
        CGPROGRAM
        //allow instancing
        #pragma multi_compile_instancing

        //shader functions
        #pragma vertex vert
        #pragma fragment frag

        //use unity shader library
        #include "UnityCG.cginc"

        //per vertex data that comes from the model/parameters
        struct appdata{
            float4 vertex : POSITION;
            UNITY_VERTEX_INPUT_INSTANCE_ID
        };

        //per vertex data that gets passed from the vertex to the fragment function
        struct v2f{
            float4 position : SV_POSITION;
            UNITY_VERTEX_INPUT_INSTANCE_ID
        };

        UNITY_INSTANCING_BUFFER_START(Props)
            UNITY_DEFINE_INSTANCED_PROP(float4, _Color)
        UNITY_INSTANCING_BUFFER_END(Props)

        v2f vert(appdata v){
            v2f o;

            //setup instance id
            UNITY_SETUP_INSTANCE_ID(v);
            UNITY_TRANSFER_INSTANCE_ID(v, o);

            //calculate the position in clip space to render the object
            o.position = UnityObjectToClipPos(v.vertex);
            return o;
        }

        fixed4 frag(v2f i) : SV_TARGET{
            //setup instance id
            UNITY_SETUP_INSTANCE_ID(i);
            //get _Color Property from buffer
            fixed4 color = UNITY_ACCESS_INSTANCED_PROP(Props, _Color);
            //Return the color the Object is rendered in
            return color;
        }

        ENDCG
    }
}

Source: https://www.ronja-tutorials.com/2020/02/11/material-property-blocks.html