InputField.selectionColor

public Color selectionColor ;

Description

The color of the highlight to show which characters are selected.

using UnityEngine;
using System.Collections;
using UnityEngine.UI; // Required when Using UI elements.

public class Example : MonoBehaviour
{
    public InputField mainInputField;

    // Changes the color of the highlight that shows what characters are selected.
    void ChangeSelectionColor()
    {
        mainInputField.selectionColor = Color.red;
    }
}