2018–2019: Widespread recognition, Japanese debut and member solo debuts
On January 4, 2018, Mamamoo released a single called "Paint Me" as a pre-release for their upcoming project series "Four Seasons, Four Colors". The goal of the series is to showcase four mini albums, each combining one color and a matching member's characteristic for each season. The group stated that they wanted to show their depth as artists and present a more mature style with the project.
Mamamoo started off the "Four Seasons, Four Colors" project with the release of their sixth EP, Yellow Flower, on March 7. Yellow Flower debuted at number one on the Gaon Album Chart, becoming their first number-one album since their 2014 debut. The Latin-inspired and dance-pop song "Starry Night" was released alongside the album, with an accompanying music video. "Starry Night" debuted and peaked at number two on the Gaon Digital Chart, earning 44.7 million Gaon Index points in its debut week. It also ranked at number six on the mid-year and 13 on the year-end editions of the Gaon Digital Chart. The song earned the group their first music recording certification since the Gaon Music Chart and Korea Music & Content Association (KMCA) introduced certifications in April 2018; in November 2018, it was certified platinum for 100 million streams, and July 2019, it was certified platinum for 2.5 million paid digital downloads.
On July 1, Mamamoo released the single "Rainy Season" as the pre-release single from their upcoming album. "Rainy Season" peaked at number two on the Gaon Digital Chart, and was nominated for Best Female Vocal Performance at the 2018 Genie Music Awards. On July 16, the group released their seventh EP, Red Moon, as the second installment in the "Four Seasons, Four Colors" series. The EP debuted and peaked at number three on the Gaon Album Chart, with 38,000 copies sold in July 2018. The EP debuted at number four on the Billboard World Albums with 1,000 copies sold, marking their best U.S. sales week, and gave the group their first-ever appearance on the Billboard Heatseekers Albums Chart, entering the chart at number 25. The Latin-pop lead single "Egotistic", released alongside the EP, peaked at number four on the Gaon Digital Chart and the Billboard World Digital Songs Sales chart, earning the group their fifth top-ten entry on the latter chart.
On August 18–19, Mamamoo held their third headlining concert, titled "2018 Mamamoo Concert 4Seasons S/S", at the SK Olympic Handball Gymnasium in Seoul. Tickets for the concert were sold out within two minutes of being available for sale.
On October 3, Mamamoo released their debut Japanese single, the re-recorded Japanese version of their 2016 single "Décalcomanie", through Victor Entertainment. The CD and digital releases of the single also include the B-side "You Don't Know Me", a pop track that serves as their first original Japanese song. "Décalcomanie" peaked at number 11 on the weekly Oricon Singles Chart, with over 9,000 copies sold. On November 29, the group returned to the Korean-language market with their eighth EP Blue;S, the third installment of the "Four Seasons, Four Colors" series. Blue;S, which focuses on member Solar, peaked at number seven on the Gaon Album Chart. The lead single "Wind Flower" peaked at number nine on the Gaon Digital Chart. On February 6, 2019, the group released the Japanese version of "Wind Flower", alongside its b-side "Sleep Talk", as their second Japanese-language single. "Wind Flower" peaked at number 16 on the Oricon Singles Chart, selling 7,000 copies.
On March 14, 2019, Mamamoo released their ninth EP White Wind, with the lead single "Gogobebe", as the fourth and final installment of the "Four Seasons, Four Colors" project. White Wind debuted at number one on the Gaon Album Chart, selling nearly 60,000 physical copies by April 2019. "Gogobebe" also saw commercial success, peaking at number five on the Gaon Digital Chart and placing at number 72 on the yearly chart. The song also peaked at number two on both the Billboard Korea K-Pop Hot 100 and Billboard World Digital Songs Sales charts. On March 27, the group announced their fourth headlining concert, titled "2019 Mamamoo Concert 4Seasons F/W", which was held at Jangchung Gymnasium in Seoul on April 19–21. The concerts served as a grand finale for the "Four Seasons, Four Colors" project.
On July 24, Mamamoo released a new promotional single titled "Gleam" in collaboration with the brand Davich Eyeglasses. The dance and pop single saw minor success, peaking at number 89 on the Gaon Digital Chart and at number 15 on the Billboard World Digital Songs Sales chart.
In August 2019, Mamamoo was announced to be joining the Mnet reality-competition show Queendom as one of six teams. The show is a "comeback battle" between six trending girl group acts in order to "determine the real number one" when all six release their singles at the same time. Queendom premiered on August 26 and ran for ten total episodes, ending on October 31. During the show's finale, the group was crowned the winner of Queendom, thus earning the prize of a full-length comeback show to be broadcast on Mnet. As part of the show, Mamamoo released various songs, including a cover of AOA's 2016 hit "Good Luck" and their original finale song "Destiny".
Following their Queendom victory, Mamamoo released their second full album, Reality in Black, on November 14, 2019, with the lead single "Hip". Reality in Black debuted and peaked atop the Gaon Album Chart, while "Hip" peaked at number four on the Digital Chart. "Hip" also debuted at number five on the World Digital Songs Sales chart and peaked atop the chart in its second week, giving the group their very first number-one hit on the chart. On February 19, 2020, Mamamoo released their third Japanese song, "Shampoo", as a digital single. The song, along with the Japanese version of "Hip", is included in the Japanese edition of Reality in Black. The Japanese edition of the album was released on March 11 and peaked at number 27 on the Oricon Albums Chart with 2,500 copies sold. In November 2020, "Hip" was certified platinum by the Korea Music & Content Association (KMCA) for surpassing 100 million streams, making it their third time achieving this feat. The song also earned a gold certification from the Recording Industry Association of Japan (RIAJ).
