When using "Gui,Color" to change the background color of a window, it will also change the color of all 0xF3F4F4 pixels, the default background color, in Indexed BMP as you can see in the image below. Both boxes are pixel-for-pixel the same, but the left one is saved as RGB, while the right one is an Indexed BMP.
My sample script below shows the issue, it creates an Image folder in the Script Directory, and downloads the two images to that folder.
#SingleInstance, Force #NoEnv ; Recommended for performance and compatibility with future AutoHotkey releases. #Warn ; Recommended for catching common errors. SetWorkingDir, %A_ScriptDir% ; Ensures a consistent starting directory. Dropbox := "https://dl.dropboxusercontent.com/u/517697/AHK%20Stuff/" IfNotExist, %A_ScriptDir%\images\ FileCreateDir, %A_ScriptDir%\images\ IfNotExist, %A_ScriptDir%\images\F3F4F4_RGB.bmp UrlDownloadToFile, %Dropbox%F3F4F4_RGB.bmp, %A_ScriptDir%\images\F3F4F4_RGB.bmp IfNotExist, %A_ScriptDir%\images\F3F4F4_Index.bmp UrlDownloadToFile, %Dropbox%F3F4F4_Index.bmp, %A_ScriptDir%\images\F3F4F4_Index.bmp Gui, Color, FF00FF Gui, Add, Picture, x5 y5 vBackground1 0x4000000, images\F3F4F4_RGB.bmp Gui, Add, Picture, x+5 y5 vBackground2 0x4000000, images\F3F4F4_Index.bmp Gui, Show, w115 h60 Return GuiClose: ExitApp