sharedMaterial.SetColor colours the entire bounding box, rather than tinting the sprite

I want to be able to easily swap the colour scheme on my game using code, rather than having a bunch of different versions of each sprite. All I want to do is tint my sprites, using their material.

When I change the 'Tint' of the material using the inspector, it works fine, tinting the sprites. But when I try to change it in code, it just colours the whole bounding box of the sprite 1 solid colour.

I'm new to using C#, so it may well be just me making a dumb mistake.

I think the issue might be the "_Color" in the SetColor code. I've seen in other posts, that it should be "_TintColor", but when I use that Unity just tells me the material doesn't have a _TintColor.

 sprite.sharedMaterial.SetColor ("_Color", newColour);

Here's my whole script, in case I've made some other stupid error in it that causes this.

     public ColourSchemes parent;
     private string currentColours;
     private Color newColour;
     private Renderer sprite;
 
     void Start ()
     {
         sprite = gameObject.GetComponent<Renderer> ();
         currentColours = "Unset";
     }
 
     void Update ()
     {
         //check for colour scheme change
         if (currentColours != parent.currentColours)
         {
             if (parent.currentColours == "Default")
                 DefaultColours ();
             if (parent.currentColours == "Midnight")
                 MidnightColours ();
             
             //set colour
             sprite.sharedMaterial.SetColor ("_Color", newColour);
         }
     }
 
     void DefaultColours ()
     {
         newColour = new Color (0xFF, 0xFF, 0xFF, 0xFF);
         currentColours = "Default";
     }
 
     void MidnightColours ()
     {
         newColour = new Color (0xFF, 0x00, 0x00, 0xFF);
         currentColours = "Midnight";
     }

'parent' is the script that handles which colour scheme is active, this script just sets one of the colours.

'currentColours' is to hold what colour scheme is active in THIS script. I use it to compare to the parent script.

'newColour is purely for clarity of reading the script. I know I could set that directly in the 'SetColor' code, but this makes it easier for me to edit at a later date.

'sprite' is my renderer, and as my colours ARE changing, just not the way I want, I guess this is working correctly.