With every new Genshin Impact update, we get new characters in the game, and to ascend them, we need to farm their ascension materials. One of them is the Rainbow Rose which is an ascension material required to ascend characters like Lyney, and it will become available in Genshin Impact 4.0 update to farm.
Their in-game description says, "A delicate and tender pink flower. Despite the name, Rainbow Roses are essentially more akin to Lilies." Since Fontaine will be the new region coming with the update, you will be able to find Rainbow Rose in it; however, it will be a difficult task to explore a massive region to farm them, and thus, here are their exact locations to save you a lot of time.
Genshin Impact: Rainbow Rose Farming Location
Rainbow Rose is a local specialty of the Fontaine and can be found all over the area and you can find all of its location marked on the map below.
You will need a total of 168 Rainbow Roses to ascend Lyney to his highest level, and as per the speculation, you can farm 73 of them in a single run. After that, you will have to wait for them to respawn to grab the next batch.
You can travel fast from one location to another by using Teleport Waypoints and the Statue of Seven to collect them quickly. In addition to this, make sure that your HP is enough to fight enemies, as you might encounter some while exploring.