You could use Monobehaviour.OnMouseEnter and OnMouseExit. The script reference page is here. Also, a good resource for searching through unity-related docs is the Custom Google Search.

Based on cjmarsh pointer I came up with the following thought I would post it here in case others came looking for similar.

Note I capture the start color but you could in most cases assume that a mats base color is white I suppose I have a few points where I am using the mat color to tent my meshes so I needed to know what it was before the mouse entered. As to the boolean testing; unnecessary to stack them like that its done for clarity in this case.

using UnityEngine;
using System.Collections;

public class MouseEventHandler : MonoBehaviour {

    private Color StartColor;
    public Color MouseOverColor = Color.yellow;

    // Use this for initialization
    void Start () 
    {
       if(renderer != null)
         if(renderer.material != null)
          StartColor = renderer.material.color;
    }

    // Update is called once per frame
    void Update () {

    }

    void OnMouseEnter()
    {
       if(renderer != null)
         if(renderer.material != null)
          renderer.material.color = MouseOverColor;
    }

    void OnMouseOver () 
    {

    }

    void OnMouseExit()
    {
       if(renderer != null)
         if(renderer.material != null)
          renderer.material.color = StartColor;
    }
}