CRT shaders are great, but they're not always appropriate for every console, seeing as handhelds' tiny LCD screens have very different characteristics, which in turn shaped the way the games for those consoles were designed. The original Gameboy handheld had these characteristics in spades, including a slow response time (manifesting as motion blur), very low resolution and a distinctive 2-color palette that ranged from yellow to green. Emulators typically represent this color palette as black and white, which can be jarring for people who spent a lot of time with the system.
Thankfully, a nice fellow named Harlequin has come to the rescue with a really great pixel shader for RetroArch that does for Gameboy displays what cgwg's CRT shader did for those displays. Like cgwg's CRT shader, Harlequin's Gameboy shader reproduces a lot of different aspects of the Gameboy display and presents a number of options to the user that are easy to modify with just a simple text editor (I use/prefer Notepad++ but almost anything will work). Unlike cgwg's, though, the Gameboy shader also uses some lookup textures (LUT) that require no text editing and can be modified using an image editor such as Photoshop or GIMP. Harlequin has included several example LUTs and I'll cover a few in screenshots.
As usual, all shots are at 4x (which is a bit small for Gameboy resolution, but oh well) and you can click the thumbnails to embiggen. All shots were taken via RetroArch with the Gambatte core unless stated otherwise.
First, here's an unfiltered shot for comparison:
'resources' folder and they're just two 64x64 squares of color that the shader pulls from to get the appropriate colors (i.e., to replace white and black in the emulator). One of the included samples is yellower and resembles the display in strong light, such as daylight:
README file and I'll include some shots here for reference.
The first one is the 'alpha' setting, which determines how dark a 'white' pixel should be. If you raise the value, everything becomes darker:
Harlequin has made some major changes to the way the shader looks and works since he first started working on it. While the current iteration is extremely convincing, I also liked an earlier version that looked more stylized. Here's a couple of shots: