If you had been there to see the very first light in the universe, this is what color it would have looked like. Let me explain (long thread warning).

After the big bang, the universe existed in an extremely hot and dense state. So hot and dense, in fact, that it was filled with a haze of super-heated plasma made up of photons, baryons (e.g. protons and neutrons), and electrons. This plasma was *very* hot.

So hot that the electrons were too energetic to bind to the baryons, and the photons, crowded by these other particles, could barely travel before getting absorbed and remitted. There was light, but you wouldn't be able to see it.

However, as the universe expanded this plasma cooled off, and electrons were able to bind to baryons to form atoms. This drastically reduced the likelihood that photons would interact with them, and these photons streamed out of the plasma-turned-gas never to interact again.

That is, until they smack into sensors here on earth. We're able to detect these primordial photons, and in fact they're all around us. They're known as the cosmic microwave background (CMB), and you may have seen maps of it such as this one from the ESA's Planck satellite.

It is called the microwave background because most of light (photons) in it have wavelengths in the microwave region of the electromagnetic spectrum. But not all of the photons do, because the CMB has what's known as a "black-body spectrum".

The black-body spectrum describes the light emitted by an object that is in thermal equilibrium at a certain temperature. If we know that temperature, we know how much of each wavelength is in the light we are measuring. We have measured the temperature of the CMB to be ~2.725K.

(an aside: The above map of the CMB actually charts temperature fluctuations in the CMB, as we point our detector at different spots in the sky. But the fluctuations are tiny! The difference between the hottest and coldest spots in the above map is only about .00001 degrees!)

Because the photons have cooled off so much, they're not as energetic as they used to be, and their wavelength has accordingly gotten stretched out. For a bunch of photons at 2.725 K, the distribution of wavelengths you expect to see looks like the plot below (credit: COBE/FIRAS)

These photons have such long wavelengths that we have can't see them, since humans only see wavelengths of a couple hundred nanometers - much smaller than millimeter microwaves. But it wasn't only this way! Remember that the exact distribution depends on the photon temperature...

and that these photons started out super hot! As it turns out, the factor by which the temperature has decreased is the same as the factor by which the universe has expanded since the photons were emitted! We can measure this factor by measuring how "stretched out" the light is.

Since these photons were emitted, we have calculated that the universe has expanded by a factor of about 1090. These photons were emitted about 380,000 years after the big bang, so between now and then the universe has expanded quite a bit!

This also means that the photons have cooled by a factor of 1090 - so if they are 2.725 K now, that means they were ~3000K when they were emitted! If we plot the distribution of wavelengths from a 3000K black-body, we see that the spectrum is shifted to shorter wavelengths.

The peak of this distribution is around 950 nm, just outside of human vision. But there is a big tail that falls into the visible spectrum, and we would have been able to see it! I've marked the rough boundaries of visible light in blue and red above, but we can zoom in here.

There's enough light there to see! But what would it have looked like? Humans are more sensitive to some wavelengths than others, with the cones in our eyes broadly responding to blue, green, and red light. We can plot the regions of sensitivity to get a sense of this (cred: wiki

Fortunately some clever folks have thought about this before, and there is a standard that links wavelength to perceived color known as the "CIE 1931 color space". This standard allows us to add up different wavelengths in our 3000K black-body spectrum with the proper weights.

When we do this we get three numbers, known as the chromaticity, that designate a specific color on the CIE chromaticity diagram (shown below - the numbers on the boundaries indicate light of a single wavelength, while the interior correspond to every perceivable mixture).

These three numbers completely define the color in a way that doesn't depend on how you device displays color. To get the final color, we need to convert these three numbers to something your device knows how to display. Fortunately there is a standard here too!

Converting our chromaticities to a sRGB value ( https://en.wikipedia.org/wiki/SRGB ), we get: R=255.0, G=186.1, B=114.5. You might be more familiar with hex codes, another way of designating colors. The hex code for this creation color is: #FFBA73

So there you have it. This is the first thing you could have seen, only 380,000 years after the big bang. Nothing, and then, suddenly an intense glow of earthy-yellow light bathing everywhere and everything, whose dim and cold afterglow we can still detect today.