Tsuuiisou 「字一色」 is a yakuman in which the hand consists entirely of honor tiles.
For this yakuman, all kazehai (wind tiles) and sangenpai (dragon tiles) can be used. So, that gives seven tile types to choose from. Given the nature of honor tiles, the different types cannot be mixed to form tile groups. Therefore, they must be collected in the form of koutsu (triplets).
This yakuman is only reasonably achievable, when in possession of at least multiple toitsu (pairs) of the honor tiles. Otherwise, players may have to resort towards yaku, like yakuhai or honitsu for attaining tenpai. Regarding tile discards, any non-honor tile is deemed irrelevant and must be discarded. This may include dora tiles. More often than not, players will call "pon" in order to claim and complete honor tile groups, until tenpai and eventually winning the hand (assuming to be the case). While it is possible to complete this hand while closed, it would seek to be in the form of a suuankou tenpai.
A specific variant to tsuuiisou is possible and can be considered as a separate yakuman, daichiisei 「大七星」, which means "Big Seven Stars". It is a variant of chiitoitsu (seven pairs). There are exactly seven types of honor tiles, which makes this difficult pattern possible but more difficult than the regular tsuuiisou. As a consequence, this variation remains as a closed only hand. This is optionally worth double yakuman.
- Tsuuiisou in Japanese Wikipedia