Check out example codes for "change trail color in script". It will help you in understanding the concepts better.

Code Example 1

TrailRenderer myTrailRenderer = GetComponent<TrailRenderer>(); 
 myTrailRenderer.material.color = myColor;
 myTailRenderer.startColor = Color.white;
 myTailRenderer.endColor = new Color (0.35f, 1f, 0.75f);

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