blue-gen is something I quickly whipped up on request, originally being a Linux alternative to halospritedoer written in C. By request, it has been released for Windows, as well.   What it does is generate TIFF sprite sheets from BMP/PNG/TGA/TIFF images to be used as input for invader-bitmap OR tool.exe. By default, it uses blue and magenta for the color plate data, but if it detects that you are using either color for your bitmaps, it will choose a different color for the color plate.   There are two programs: blue-gen and Blue Genstone. blue-gen is a command-line program that does all of the work (see the readme for the exact usage). Blue Genstone is a graphical user interface for blue-gen. You can use blue-gen by itself, but if you use Blue Genstone, ensure that blue-gen as well as any dependencies (e.g. DLL files) are the same folder.   Here's the command-line program in action:   I also made a GUI for it as a proof of concept:   And here's the Windows version which has been slightly updated:   Download (Windows 64-bit): blue-genstone-r2.7z Source code: https://github.com/Kavawuvi/blue-gen