* The red will likely influence the final colour. Red is a very strong pigment and difficult to completely cover up. Depending on how vibrant the red is and how well the brown dye takes, you'll likely end up with some variation of auburn, reddish-brown, or a warm brown with red undertones.
* The intensity of the red matters. If the red is faded and more of a pastel, you might have more luck achieving a browner result. However, if it's a bright, vibrant red, it will be much harder to cover.
* The specific brown dye will affect the result. Some light brown dyes have more warm (red/gold) tones in them, while others are more neutral or cool-toned. A cooler-toned light brown dye would be a better choice if you're trying to neutralize the red, but even then, complete neutralization is unlikely.
* Porosity and damage of the hair. Dyed hair tends to be more porous, which means it can absorb colour unevenly. The red-dyed hair will likely absorb the brown dye differently than virgin hair, potentially leading to an uneven result.
In summary, expect a warm, reddish-brown result. If you want a true light brown, you'll likely need to take steps to neutralize the red tones in your hair before applying the brown dye. This could involve using a colour remover specifically designed to remove red pigments, followed by a toner to neutralize any remaining red, before finally applying the light brown dye. It's best to consult a professional hair stylist for guidance, as correcting colour issues can be complex.