MAC Blue Fountain is a neutral-toned, medium-dark blue with a matte finish. It is a limited edition eyeliner that retails for $17.50.
MAC Blue Fountain Fluidline ($17.50 for 0.10 oz.) is described as a "dark green blue." It's a medium-dark blue with subtle, cool undertones and a mostly matte finish. It had semi-opaque color payoff in a single stroke, and it was buildable to mostly opaque coverage in two to three layers. On me, it lasted for twelve hours without fading or migrating. MAC Mountain Air (P, $20.00) is more shimmery, darker (95% similar). Lethal Cosmetics Threshold (P, $15.50) is warmer (90% similar). MAC Blue Peep (P, $16.00) is lighter, warmer (90% similar). See comparison swatches / view dupes side-by-side.