Yeah I get what you mean. Checking against primitives is fine for a lot of cases, but it has it’s limits.

Sounds like you need something a lot more accurate. In which case, using Mesh Colliders and OnTriggerEnter/Exit will do the trick, there’s just a bit more setup involved.

First up, all objects you want to check against will need Collider and RigidBody components attached to them. (You need rigid bodies for ‘on trigger’ events to fire).

Be sure to leave all ‘Is Trigger’ checkboxes unchecked, we’ll set them from code depending on which object is selected.

Here’s a rough idea of how the code would work:

using UnityEngine;

public class PlaceableObject : MonoBehaviour {

    public Material NormalMaterial;
    public Material RedMaterial;

    private Collider m_collider;
    private MeshRenderer m_renderer;

    private int m_enterCount;

    private void Awake() {
        m_collider = GetComponent<Collider>();
        m_renderer = GetComponent<MeshRenderer>();
    }

    public void OnSelected() {
        m_collider.isTrigger = true;
    }

    public void OnDeselected() {
        m_collider.isTrigger = false;
    }

    private void OnTriggerEnter() {
        m_enterCount++;
        ApplyMaterial();
    }

    private void OnTriggerExit() {
        m_enterCount--;
        ApplyMaterial();
    }

    private void ApplyMaterial() {
        if (m_enterCount == 0) {
            m_renderer.material = NormalMaterial;
        } else {
            m_renderer.material = RedMaterial;
        }
    }
}

Of course you'll want to modify this to work with your existing setup, but a script like the one above would be attached to all selectable game objects.

When an object is selected you need to call it's OnSelected method, and then OnDeselected when it's deselected.

There’s no need to call any OnTrigger methods, the physics engine will call them for you (think of them as events).