These shaders anti-alias the sharp pixel edges to maintain the sharpest possible image without uneven sizes (which manifests as "shimmering" during scrolling). "pixellate" or one of the later versions with faster performance (e.g., sharp-bilinear) or more mathematically correct processing (e.g., bandlimit-pixel). The best compromise for sharp pixels without these drawbacks is to use a shader from the 'interpolation' directory, such as the o.g. Unfiltered, nearest neighbor is as sharp as you can get, but it can cause uneven pixel sizes with non-integer scale factors, which means you have to use less of your screen and/or suffer from inaccurate aspect ratios. It's all a taste thing, but as time goes on (and fewer and fewer people have actually seen these games in their original context), users are more accepting of and express a preference toward ultra-sharp pixels.
Sometimes a CRT scanline shader looks even better than a LCD/Grid shader, it gives a SNES-style look that's often even better than a GBA shader. Like someone said, CRT Shaders also make GBA games look good. Though it depends on the game and the release date, and on which GBA revision hardware it was made for. Keeping it too small makes some games unplayable. Choose the correct overlay preset for the system youre playing. Settings > On-Screen Display > On-Screen Overlay > Overlay Preset >. I tend to play on smaller screens (phones and such), so there often isnt much stretch or extra space. To enable an overlay for a system, while in a game: 'Menu' + 'X' to open Quick Menu > 'B' to back out to RetroArch Main Menu >. The color palettes were made to be viewed through the dull screen, which is why they over-saturated them. They fill in the black on the outside and make it to where you dont have to stretch out the low res GBA image to fit, while still having a big enough image to see while playing. The config files files can be found in: RETROPIEconfigsgbretroarch.cfg RETROPIEconfigsgbcretroarch. You will need to edit the retroarch.cfg file for gameboy/gameboy color to make sure that the viewport matches the window in the borders. You mentioned you like the saturated colors, but keep in mind it's usually not what the artists intended. Ive only tested them on windows so far but they should work on retropie just the same.They're like scanline filters but a grid mimicking LCD handhelds. Combined with that GBA-Color, you want LCD 3x or LCD 2x, sometimes these are listed as "Grid".You want GBA-Color shader, assuming the core feature for adjusted GBA colors is turned off.Bilinear filtering makes pixel art look horrible, though many people don't realize this (usually because they're comparing it with raw unfiltered pixels, which is also wrong and bad).