Excerpt taken from: ClassReptilia

Unlike the t-negative albino boas where melanin remains absent throughout their lives, t-positive boas will produce a reduced amount of melanin as they mature. T-positive boas have functional tyrosinase enzymes required to synthesize melanin, but they lack a subsequent enzyme required to complete the melanin-synthesis pathway normally.
He sums it up nicely. I knew what it was, but I didn't know how to explain it right. Hope this helps.