It's actually very different from plant to plant what they need in order to get that amazing red colour. We think of the red colour as being a sign of health, but in nature it's a sign of stress. But for most plants think of the red colour as humans getting a tan. It's actually to protect from the light. which is why light is often the limiting factor when dealing with red colours.
But as stated above, remember that your tank needs to be stable. Your plants needs the right temperature, light and the right mix of nutrients (think of CO2 as a nutrient)
If you only raise one bar it won't help the plants one bit, because the lowest bar will always be the limiting factor. However it will help the algae who aren't as demanding.
Low light and high CO2 is just a waste of CO2, because the plants cant utilize it's full potential.
High lights and no CO2 and low nutrients is going to give your plants damage, as the light will give them `sun burns´ but they lack CO2 and nutrients to repair the damage.
You see?