Why Bit Depth Still Matters for Web Images
Bit depth may feel like a relic from the dial-up era, when we agonized over 2-, 4-, or 8-bit color in GIFs because every kilobyte meant longer load times. But even on broadband, every kilobyte still counts—especially for mobile users on slower connections. And contrary to what you might assume, modern optimization tools don't always handle color depth reduction for you automatically.
This is particularly true for PNGs. Many image editors default to saving PNGs at 2^24 color depth "just in case." That's reasonable for photographs (though you'd typically serve those as JPEG or WebP anyway), but for logos, icons, and UI graphics, it's wildly excessive—you're unlikely to use more than a few hundred distinct colors.
Indexing Colors: A Simple Win
In Acorn, reducing the bit depth in the export dialog has proven highly effective. By indexing colors to a palette of 256 or fewer values, image sizes can be cut by more than 80% with no visible quality loss—again, assuming you're not dealing with photographs.
Here's a concrete example:
At full color depth, the sample PNG weighs about 379KB. Restricted to just 32 colors, it drops to 61KB. After running both through ImageOptim, the sizes shrink further: 359KB and 48KB respectively. That's roughly an 85% reduction simply by lowering the color depth. If the image turns out to need more colors later, you can regenerate it with a 64-color palette—still a huge savings at 73KB.
Manually adjusting color depth for each image is certainly more labor-intensive than running a batch optimization script. But in practice, the payoff in reduced image weight and faster page loads makes it well worth the effort—and that ultimately benefits the user experience more than any automated tool can.
