Using a Slider to change a color to pre-defined colors based on slider value

[MarksTeq]

You never actually called the function:
public void GoodSlider(float value)

So just add the line to update and it should work:

    void Update()
    {
        GoodSlider(this.GetComponent<Slider>().value); //Add this
        images = gameObject.GetComponentsInChildren<Image>();
        foreach (Image image in images)
        {
            if (Goodvalue >= 0f && Goodvalue <= 14f)
            {
                image.color = Colors[0];
            }
            if (Goodvalue >= 15f && Goodvalue <= 34f)
            {
                image.color = Colors[1];
            }
            if (Goodvalue >= 35f && Goodvalue <= 63f)
            {
                image.color = Colors[2];
            }
            if (Goodvalue >= 64f && Goodvalue <= 100f)
            {
                image.color = Colors[3];
            }
        }
    }

Side note (untested) : Why don’t you just make your slider go from 0 to 3 for each of the colours? (You can even tick [Whole Numbers] to snap to those numbers) because then you can just:

image.color = Colors[(int)this.GetComponent<Slider>().value];

or

image.color = Colors[(int)Goodvalue];

Instead of the ifs…

An even more efficient way would be to create a ChangeColor method and call it on the sliders OnValueChanged method:

    public void changeColor()
    {
        GoodSlider(this.GetComponent<Slider>().value);
        images = gameObject.GetComponentsInChildren<Image>();
        foreach (Image image in images)
        {
            image.color = Colors[(int)Goodvalue];
        }
    }