https://learn.microsoft.com/zh-tw/dotnet/api/system.windows.forms.imagelist.transparentcolor?view=netframework-4.8.1&viewFallbackFrom=windowsdesktop-10.0
ImageList.TransparentColor Property (System.Windows.Forms) | Microsoft Learn
Gets or sets the color to treat as transparent.
property systemwindows formsimagelistmicrosoftlearn