RGBAそれぞれの値を個別に変更したい場合(例ではA)

ColorChangePerComponent.cs

using System.Runtime.InteropServices;
using UnityEngine;

public class ColorTest : MonoBehaviour
{
    [StructLayout(LayoutKind.Explicit)]
    public struct Color32RGBA
    {
        [FieldOffset(0)]
        public byte R;
        [FieldOffset(1)]
        public byte G;
        [FieldOffset(2)]
        public byte B;
        [FieldOffset(3)]
        public byte A;

        [FieldOffset(0)]
        public Color32 color;
    }

    Material mat;
    Color32RGBA rgba;

    void Start()
    {
        rgba = new Color32RGBA();
        rgba.color = new Color32();
        mat = GetComponent<Renderer>().material;
        mat.color = rgba.color;
    }

    void Update()
    {
        if (rgba.A == 255)
            rgba.A = 0;
        else
            rgba.A++;
        mat.color = rgba.color;
    }
}