The GoPro HERO6 Black, HERO5 Black, HERO5 Session, as well as the HERO4 Black and Silver models all offer a choice between two color modes in the photo mode: GoPro and Flat. So what's the difference and when would you use them?
These are options you're only going to find in the current range on the HERO4 Black and HERO4 Silver. And you'll only see them if you turn the Protune option on, which is accessible under the Photo mode settings and is separate from the Protune options for video.
Protune is GoPro's name for a group of settings that provides more fine-tuned control over several aspects of the shooting such as sharpening, color, ISO, etc. With some exceptions, as a general rule of thumb, you probably only want to turn Protune on if you plan to do further processing in post-production. With some of the settings, if you just share the footage or photos without doing any post production they can actually end looking worse than if you'd left Protune off.
The color mode setting is a perfect example. If you shoot with the default GoPro color mode, the image will be vibrant and contrasty. If you shoot with the Flat color mode, the image will be, well, flat and the colors will look washed-out.
Here are some examples. The first one was shot with a HERO5 Black:
Here are some more examples shot with the HERO4 Silver and Black cameras.
As you can see, the most obvious difference is in the contrast. That is also why the colors appear less saturated. What it's trying to do is retain more detail throughout the image without the risk of losing details in the shadows or blowing out the highlights.
The result is an image that actually retains more image data but doesn't look as impressive.
Which to Use?
When Protune is turned off, the GoPro color mode is what's applied automatically. It gives that contrasty, colorful, punchy look.
So why would you ever want to use the Flat mode if it makes the photos look worse? If—and really only if—you plan to process the images later in something like Lightroom or some other image processing app where you're going to adjust things like exposure and contrast. Or if you want to retain that option for later.
There's nothing at all wrong with processing images shot with the GoPro color modes--it's just that the Flat mode gives you a bit more flexibility in preserving tones. The GoPro HERO4 lines don't have RAW mode—the HER05 models do—so this is a step in that direction while still using JPG.
If you plan to share the photos with minimal or no processing, you're much better off leaving the color setting on the default GoPro mode—your images will nearly always look better.
An exception would be if you're deliberately aiming for a low-contrast, washed-out look, but that's definitely a niche approach and doesn't apply to the vast majority of photos taken with a GoPro.
Of course, the standard GoPro look might not be what you're after. But if you're using Lightroom and want a preset that takes images shot with Protune's Flat setting and processes them to approximate the look of the GoPro color setting, you're welcome to try this preset I put together. It's not perfect, and I'll likely tweak it a bit yet, but it's a reasonable starting point. The images sometimes come out a touch lighter than the real GoPro-setting equivalent, which is easily fine-tuned by dropping the Exposure slider ever so slightly and moving the Blacks slider slightly to the left.
If you're not sure how to install a preset, here's a step-by-step guide.