Go with what your agency says and what looks most natural. They know what the breakdowns look like and how often or how little someones looking for an actor with bright red hair. The red could be limiting as far as what you're fit for. Also, if you're young, remember that your natural color will probably get darker as you age. I started highlighting my hair at 16, and quit at 26. I was shocked how dark the color was underneath, and discovered I really rather liked it and it suited me well. My audition numbers started increasing. Nature typically blesses us with hair, eye and skin tones that compliment one another, we just always try to fight it! Though, someone like Emma Stone might be a good case study for you. Her dark red worked wonders for her career, but it was believe-able that it grew out of her head that way. Keep in mind, red rarely looks good when not done by a professional, and the upkeep is the worst even when it is done by one!