ColourPop Guess is a slightly cool-toned, very dark purple with a matte finish. It is a permanent lipstick that retails for $6.50 and contains 0.11 oz.
ColourPop Guess Ultra Matte Liquid Lipstick ($6.00 for 0.11 fl. oz.) is described as a "dark blackened violet." It's a blackened purple with subtle, cool undertones and a matte finish. It had semi-opaque pigmentation (one of the least pigmented shades I've encountered in the line-up!) that dries down to a mostly matte finish. Unfortunately, as it is a very dark color, that sheerness also came with unevenness. It wore for eight hours without flaking, and it was moderately drying after six and a half hours of wear. Obsessive Compulsive Cosmetics Stud (DC, $17.00) is lighter, glossier (90% similar). Too Faced Melted Villain (P, $21.00) is darker, glossier (85% similar). Fenty Beauty Undefeated (P, $24.00) is lighter, brighter (85% similar). Bite Beauty Black Truffle (DC, $26.00) is darker, glossier (85% similar). MAC Which Witch (LE, $19.00) is warmer, glossier (85% similar). MAC Smoked Purple (P, $19.00) is warmer (85% similar). Maybelline Concrete Jungle (P, $7.49) is lighter, glossier (80% similar). Urban Decay Vanity (P, $18.00) is darker, brighter (80% similar). NARS Wild Night (P, $26.00) is warmer (80% similar). See comparison swatches / view dupes side-by-side.
FURTHER READING: Formula Overview, which is a detailed review of the range as a whole.