简单暴力一点啊，不多废话直接上步骤

1. class内建三个float分别代表HSV(Hue,Saturatoin,Value), 同时用三个slider控制value

2. Canvas内拉个白图，通过喜好的方式reference和HSV reference一下

3. 通过Color.HSVToRGB(float H,float S,float V)函数创建新的Color，和想要改变颜色的Color互相更新一下就ok了

UMA部分拉到最底下

代码部分：共有两个class （我知道public reference不是好习惯……我会克服懒惰的）

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

public class ColorPicker : MonoBehaviour {
    public Color colorPicked;
    public float H = 0;
    public float S = 0;
    public float V = 0;
    public Image colorShower;

    private void Start()
    {
        colorPicked = Color.HSVToRGB(H, S, V);
    }

    public float HSVChangeHolder(int index,float value)
    {
        if (index == 0)
            ChangeH(value);
        else if (index == 1)
            ChangeS(value);
        else if (index == 2)
            ChangeV(value);
        return index;
    }

    void UpdateColor()
    {
        colorPicked = Color.HSVToRGB(H, S, V);
        colorShower.color = colorPicked;
    }

    public void ChangeH(float value)
    {
        H = value * 1;
        UpdateColor();
    }

    public void ChangeS(float value)
    {
        S = value * 1;
        UpdateColor();
    }

    public void ChangeV(float value)
    {
        V = value * 1;
        UpdateColor();
    }
}

含蓄的分割

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

public class ColorPickerSlider : MonoBehaviour {
    public Slider slider;
    public ColorPicker colorPicker;
    public int HSVIndex;

    private void Start()
    {
        slider.onValueChanged.AddListener(delegate { DelegateChange(); });
    }

    void DelegateChange()
    {
        colorPicker.HSVChangeHolder(HSVIndex, slider.value);
    }
}

含蓄的二分割

void UpdateColor()
    {
        colorPicked = Color.HSVToRGB(H, S, V);
        colorShower.color = colorPicked;
        UMA.SlotData slot = human.GetComponent<UMA.UMAData>().GetSlot(slotIndex);
        List<UMA.OverlayData> list = slot.GetOverlayList();
        foreach (var v in list)
        {
            v.SetColor(0, colorPicked);
            v.SetAdditive(0, colorPicked);
        }

        //The below is super important!!!Wont work without setting dirty!!!

        human.GetComponent<UMA.UMAData>().isTextureDirty = true;
        human.GetComponent<UMA.UMAData>().Dirty();
    }