Choose color/palette swap?

What the hell does that code do?!...

1. Firstly we must somehow fetch the sprite that was clicked on. To do this I use a raycast from the camera to the mouse position. This requires your sprite to have a collider! (it can be a trigger). If there isn't a collider on your sprite gameObject the raycast will return nothing or whatever is behind your sprite.

2. Once we have the gameObject of your sprite we must get the actual spritRender component of it using GetComponent()

3. now that we have found a GO with a spriteRenderer that we want to recolour we must find which pixel was clicked, this is the difficult part! (or at least I don't know an easier way unless you know the shader is not effected by lighting).

4. If the sprite is known to never be rotated then most of the code in GetPixelColor is obsolete, though if you have sprites (like arms, legs) that are being rotated then GetPixelColor is able to work out how far from the bottom corner of the sprite the hit point is in Cartesian coordinates that are relative to the sprites axis (this assumes that the box collider fits the sprite exactly, which is the default when you add a new box collider).

5. Once we have the vector from the bottom corner of the collider to the hit point, relative to the sprite. We can use this information combined with the size of the piece of texture used by the sprite, to find the exact coordinate of the pixel clicked in the main texture atlas.

6. Once we have the pixel we wish to recolour we need to find others of the same or similar colour which is done in 'RecolourTexture'

7. RecolourTexture takes several arguments, the ones you may wish to change which are given yourself are 'buffer' which is an integer which allows all colours that are a 'distance' of buffer away from your selected colour to also be coloured (where 255 will colour everything). And 'wholeAtlas' which you should give a value of true or false.

8. If wholeAtlas is set to true, the recolour will be applied to the entire texture atlas, if it is set to false then only the section of the texture used by the clicked sprite will be affected.

9. The last variable you will wish to change is 'texColor' which is the colour that you are swapping in, in place of the selected colour. As your question says, 'Let's pretend I already know how to make a 255 RGB ramp and display it' you would use your result from that colour ramp and pass those values to 'texColor', as it is, you should be able to change the colour from the instpector.

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   The Code

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   private var screenTexture : Texture2D; private var mousePos : Vector2; private var screenColor : Color; var texColor : Color; private var colorPreview : boolean = false; var buffer : int = 20;

   function Start(){ screenTexture = new Texture2D(1, 1); }

   function RecolourTexture(point : Vector2, sprite : Sprite, buffer : int, wholeAtlas : boolean){ var texture = sprite.texture; var spriteRect = sprite.rect; var selectedColor = texture.GetPixel(point.x, point.y); var correctedBuffer = (parseFloat(buffer)/255.0); var colorDiff : float; var pixels : Color[];

     if(wholeAtlas){
            pixels = texture.GetPixels(0, 0, texture.width, texture.height);
            for(pix in pixels){
                colorDiff = Vector4(
                    pix.r-selectedColor.r,
                    pix.g-selectedColor.g,
                    pix.b-selectedColor.b,
                    pix.a-selectedColor.a).sqrMagnitude;
                if(colorDiff <= 4*(correctedBuffer*correctedBuffer)){
                    pix = texColor;
                }
            }
            texture.SetPixels(0, 0, texture.width, texture.height, pixels);
            texture.Apply();
        }else{
            pixels = texture.GetPixels(spriteRect.x, spriteRect.y, spriteRect.width, spriteRect.height);
            for(pix in pixels){
                colorDiff = Vector4(
                    pix.r-selectedColor.r,
                    pix.g-selectedColor.g,
                    pix.b-selectedColor.b,
                    pix.a-selectedColor.a).sqrMagnitude;
                if(colorDiff <= 4*(correctedBuffer*correctedBuffer)){
                    pix = texColor;
                }
            }
            texture.SetPixels(spriteRect.x, spriteRect.y, spriteRect.width, spriteRect.height, pixels);
            texture.Apply();
        }
    }
    
    function GetPixelColor(hit : RaycastHit, spriteRenderer : SpriteRenderer){
        var point = hit.point;
        var col = hit.collider;
        var go = col.gameObject;
        var up = go.transform.up;
        var right = go.transform.right;
        var forward = go.transform.forward;
        var hitVector = point - go.transform.position;
        var projectedVector = Vector3(
            Vector3.Project(hitVector, right).magnitude,
            Vector3.Project(hitVector, up).magnitude,
            Vector3.Project(hitVector, forward).magnitude);
        var scalerVector = Vector3(
            Vector3.Dot(right, Vector3.Project(hitVector, right))
                /Mathf.Abs(Vector3.Dot(right, Vector3.Project(hitVector, right))),
            Vector3.Dot(up, Vector3.Project(hitVector, up))
                /Mathf.Abs(Vector3.Dot(up, Vector3.Project(hitVector, up))),
            Vector3.Dot(forward, Vector3.Project(hitVector, forward))
                /Mathf.Abs(Vector3.Dot(forward, Vector3.Project(hitVector, forward))));
        var realVector = Vector3.Scale(projectedVector, scalerVector)
            +(0.5*Vector3.Scale(col.size, go.transform.localScale));
            
        var sprite : Sprite = spriteRenderer.sprite;
        var spriteRect = sprite.rect;
        var pixelPos = Vector2(
            spriteRect.x + Mathf.Clamp(Mathf.Floor(realVector.x*spriteRect.width), 0, spriteRect.width),
            spriteRect.y + Mathf.Clamp(Mathf.Floor(realVector.y*spriteRect.height), 0, spriteRect.height));
        
        RecolourTexture(pixelPos, sprite, buffer, false);
    }
    
    function Update(){
        mousePos = Vector2(Mathf.Clamp(Input.mousePosition.x, 0, Screen.width-1), Mathf.Clamp(Input.mousePosition.y, 0, Screen.height-1));
        if(Input.GetButtonDown("Fire1")){
            colorPreview = true;
        }
        
        if(Input.GetButton("Fire1")){
            //colorPreview = false;
            
            var ray = Camera.main.ScreenPointToRay(Input.mousePosition);
            var hit : RaycastHit;
            if (Physics.Raycast (ray, hit)) {
                var col = hit.collider;
                var spriteRenderer = col.gameObject.GetComponent.<SpriteRenderer>();
                if(spriteRenderer != null){
                    GetPixelColor(hit, spriteRenderer);
                }
            }
        }
        
    }
    
    function OnPostRender(){
        screenTexture = ScreenColor(mousePos, screenTexture);
    }
    
    function ScreenColor(mousePos : Vector2, tex2 : Texture2D){
        var tex = new Texture2D(1, 1);
        tex.ReadPixels(new Rect(mousePos.x, mousePos.y, 1, 1), 0, 0);
        var col = tex.GetPixel(0, 0);
        col.a = 1;
        screenColor = col;
        tex.SetPixel(0, 0, col);
        tex.Apply();
        return tex;
    }
    
    function OnGUI(){
        if(colorPreview){
            GUI.Box(Rect(mousePos.x-12,Screen.height-mousePos.y-12,14,14), "");
            GUI.DrawTexture(Rect(mousePos.x-10,Screen.height-mousePos.y-10,10,10), screenTexture);
        }
    }
   
   
   

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Don't expect many people to write code like this for you, I was simply interested enough to want to find a solution myself :)

Hope that helps!

Scribe