The distance of migration is inversely correlated with the molecular weight of the protein (small proteins move faster through the gel than large proteins do and therefore migrate further down the gel).
Shellac is the most expensive of the gel nail polishes on the market, and the bottles are half the size of the competition. The higher cost for the salon owner is one reason Truong believes other salons might not correct you if you ask for Shellac but choose a different gel polish when picking your color.
Although gel manicures are generally only done in salons, OPI and CND are both pushing at-home versions. The polishes are expensive (think $20 a bottle) and the U.V. lights used in salons can cost $600. (The at-home versions start at around $40.) A gel manicure can last up to 20 days.