The Dark Eyes: The European Red Squirrels Eyes, are almost completely black, which many would associated with night vision or low light capacities. Which is not the case here. Night vision squirrels like the flying squirrel family, are primary nocturnal. But not the European Red Squirrels. They wake up at sunrise, which is where the sun is extra bright, especially up north. They use this brightness to their advantage, to minimize their visibility from birds of prey. Blending in on the forest floor, as a red orange leaf. But this also means limited visibility for themselves, unless they have some sort protection against the bright, morning sun. To solve this, red squirrels have developed a yellow and brown pigments in their eyes, you might even call this, builtin sunglasses This protects their eyes from the bright uv lights, as well as protection from flash burns doing the winter, also known as welders eyes, white out or snow blindness. Doing the winter, the snow will reflect the sun's bright beams and every little snowflake will act as a magnifying glass.
Range and capabilities: Squirrels eyes are placed in a pray position, which means they are located on the sides on the head, instead of predatorial vision, which is front facing. This gives them a very wide range of vision, about 240 degree visibility. In contrast to humans, which can only focus on the very center of their sight. But, they can't determine the distance, until they turn their heads, using both eyes. So if a squirrel turns their back to you, they trust you and count on you to keep their back safe for them. These pigment also serves as an anti glare effect, giving them perfect vision, even if something is reflecting the sunlight, directly into their eyes.
Disadvantages: These pigments gives them very poor eyesight doing low light. Imagine wearing sunglasses at night. Another downside is that, colors will be muted quite significantly, especially doing low light.
Conclusion: These effects and capabilities are also seen in other squirrel species, like the American gray squirrels, Although not as perfected as the European Red Squirrels. The American gray squirrels eyes are more universal in their use, giving them better low light visibility, but worse anti glare and bright light vision.
Side notes: To be able to see the European Red Squirrels true eyes, you have to capture the photo at a very specific angle, in bright sunlight, as well as manipulating the contrast and brightness of the photos.