I like to wear a visor under my helmet. Basically a short visor like a cycling cap without the cap part. I ride with both black and white, and I feel like I like the black better, but I also think it looks better, which is weird I know, but don’t judge me because I am acknowledging that I might be biased.
Other than black absorbing more light and thus heat, is either better or worse for vision, including fatigue?
And if so, does that only apply to the underside?
Black is likely to be opaque, and properly block sunlight. White is likely to be at least slightly translucent, and glow. I'd rather not have that illuminating the edge of my sunglasses. Fabric cap brims can be both white and opaque, as the stiffening material can block the light. It's not clear whether yours are fabric-covered or just plastic.
Most helmet detachable peaks are black. I have in the past had a red one and I really didn't like the blinking red glow at the top of my field of vision when riding in and out of shade. It was too much like brake lights ahead, despite being a different shade of red and too broad.
The heat shouldn't matter too much, as most of it will radiate away.
Choosing kit based on looks is fine, and perfectly normal. Personally I prioritise function but definitely consider appearance. And black will certainly look better than white once you've been splashed with muck or splattered with insects. Black cap brims can show a salty tide-mark from sweat, but that washes out.
I have a black helmet. Originally I thought it could make my head too hot, but that hasn't been my experience.
Then I did some calculations.
The helmet has a surface area of maybe 0.15 square meters. That can in theory absorb 150 watts of heat. However, it's similar to a hemisphere (although the radius is different in different directions), so considering that sun shines only from a certain angle, it's more like 0.05 square meters or 50 watts of heat.
The cushioning is made from 2.5 cm thick expanded polystyrene. Expanded polystyrene has heat conductivity of 0.04 W/(m*K)
So if we assume the surface gets so hot it's enough to boil water, and your head is 37 degrees Celsius, it's 63 K * 0.15 m2 * 0.04 W/(m*K) / 0.025 m = 15 watts of heat transferred to your head. However, I have never been able to boil water using my black helmet so clearly the assumption of 100 degrees Celsius temperature at the helmet surface is incorrect.
And here's why it can't boil water. The 50 watts of heat it absorbs is carried away by the wind. The frontal cross section is 0.05 m2, so at 25 km/h or 6.94 m/s, 0.347 m3 of air pass over it every second. That's about 0.42 kg of air every second. The venting holes are made such that most of the air goes via these, some of the air can flow over the surface of the helmet. Air has a heat capacity of 1000 J/(kg*K) at constant pressure so every second, 420 Joules of heat is extracted per Kelvin of temperature difference.
So to extract 50 watts (50 Joules per second), we need 0.12 Kelvins of temperature difference.
So now we know how hot the helmet exterior is. It's 0.12 Kelvins hotter than your head, not enough to boil water. So 0.028 watts of heat gets conducted to your head via the polystyrene.
If you produce 150 watts of power at 25% efficiency, that's 450 watts of discarded heat. Head surface area of a non-obese adult is 9% of the total body surface area, so if we assume that each body part is proportionally doing its share to discard waste heat, your head is discarding waste heat at a rate of 40.5 watts.
So 0.028 watts / 40.5 watts = 0.07% approximately.
A black helmet in full sunshine therefore makes your head 0.07% hotter than it would be barely from riding the bike.
And remember here you are considering, not a helmet (which can be either black or white), but a visor. A visor doesn't even touch your head, it's just something over your eyes.
I would select a black visor. Black absorbs sunlight, white reflects sunlight albeit in a mostly diffuse manner and not specularly. Therefore, black would be best for your vision.
As for helmet, if I had a choice, I would select a bright green helmet because it's the most visible in traffic. However, my helmet selection criteria are (1) Snell certification, (2) in-mold technology, (3) MIPS, and looking at the current Snell certified helmets list makes me really sad. Out of general-purpose helmets, Specialized was the only company certifying its helmets to Snell standards. It has abandoned the Snell standard. So every Specialized helmet that's a recent design has a high chance of not meeting the Snell standards. Therefore, it looks like I can no longer purchase reasonable helmets.
I use a Black brim on all my caps/helmets.
Day time - black prevents bright sunlight coming through - a white visor needs to be thicker to provide shade to the eyes.
Night time - white can glow more when a car headlight illuminates your face. This ruins your night vision faster and takes longer to recover.
Sweat - harder to stain black cloth compared to lighter colours. White shows it up badly and washing doesn't necessarily remove the marks.
Oils and road dirt - If you end up touching a chain or tyre or other dirty surface, this can be transferred to your peak. A black peak does not show dirty marks so much as a white one.
The rest of the helmet and cap/casket can be any colour you like - I'm specifically referring to the underside of the visor nearest your eyes.
