A tampon is treated with the stringency of an internal bedikah; therefore, a stain on a tampon would be judged more stringently than a stain on toilet paper – see the article on stains (ketamim) for further explanation.
So, if the discharge you describe has any hint of a problematic color, the tampon might be more trouble than it's worth, although using it would be permissible.
Your best bet to catch the discharge would be to wear a black panty liner.
If you consistently have this flow prior to your period, this may actually be the proper time from which to count your veset. It would be best to discuss this further with your rabbi.