The colorspace of a pixel determines how the numerical values of the pixel are interpreted. This is different than the pixel format which specifies how the different channels and their bits are arranged in memory.
Linear vs sRGB color spaces
The two colorspaces we'll focus on are the linear and sRGB ones. Both can have the same pixel format, say BGRA32, which means that there each blue, green, red, and alpha channel each have 8 bits. The way these bits are interpreted, however, can differ.
In a linear colorspace, a doubling of a value leads to a doubling of the intensity. Adding two colors together becomes as simple as adding the values of each channel together, which is straightforward.
sRGB on the other hand is non-linear, meaning that doubling any value will not double the intensity of that value.
Why even have non-linear colorspaces, if the linear one is so intuitive?
The simple answer is that the human eye does not perceive light linearly. It is better able to discern differences in light in low-light environments than bright environents, for instance. Therefore non-linear colorspaces such as sRGB have the ability to better express images as we would see them in real life.
What this means for you
The Screenshot protocol outputs images in the sRGB colorspace. This means that if you are using it for non-saturated pixel comparison tests, you should convert the screenshot from sRGB into a linear colorspace. Non-saturated pixels means one or more channels are non-0 or non-1.
If you are using the image to create GPU resources, however, this conversion is unnecessary and you can use the output as VK_FORMAT_*_SRGB.