In your Plants_Cultivated_Flowers.xml file look for <texPath>Things/Plant/Violet</texPath>. Even though this is the correct order for your mod folder it is identical to the pathing that Tynan uses for Rimworld so the game isnt looking in your mod folder. Place your Violet.png into a folder named anything but Plant and you should be good
EDIT: For some more info. As far as I know only the first set of folders in your mod folder need to be named specific names (ex. About, Textures, Patches, Defs, etc) All child folders can be named what best suits you, following the Core folder path is just a formality
EDIT: For some more info. As far as I know only the first set of folders in your mod folder need to be named specific names (ex. About, Textures, Patches, Defs, etc) All child folders can be named what best suits you, following the Core folder path is just a formality