Short answer
It depends entirely on which sense of 'colour' you mean, and almost every confident answer online has quietly picked one and presented it as the answer. In terms of light, white is all visible wavelengths together, so it is not a single colour. In terms of pigment, white is a real, manufactured, purchasable colour. In everyday language it plainly is one.
White is not a wavelength. White light is what you get when the whole visible range arrives together in roughly balanced proportions, which is why a prism can separate it into a spectrum. On that definition white is a mixture rather than a colour.
White paint is a pigment like any other — titanium dioxide, historically lead white — sold in tins, mixed into other colours to lighten them, and available in dozens of distinguishable versions with different undertones. Denying that white is a colour here would be an odd thing to tell someone standing in front of a paint chart.
On a display, white is produced by driving the red, green and blue subpixels together at full output. It is the sum of the primaries rather than one of them — the additive counterpart of the physics answer.
People choose white, specify white, and distinguish confidently between warm and cool whites. In any practical conversation about decorating, clothing or design, treating white as a colour is simply how the word is used.
The disagreement is not really about white. It is about the word 'colour', which does duty for two different ideas. In the physical sense, a colour is a wavelength or a narrow band of them, and white is definitionally a mixture. In the perceptual and practical sense, a colour is any distinguishable visual appearance you can name, choose and reproduce — and white qualifies without difficulty. Both senses are legitimate and long-established. The mistake is not picking one; it is pretending the other does not exist.
Even accepting white as a colour, there is no single white. Every white surface has an undertone, because it is impossible to reflect all wavelengths perfectly evenly: whites lean yellow, pink, green, blue or grey depending on their pigment mix. This is immediately obvious to anyone who has held two 'white' paint samples together, and it is why a brilliant white ceiling can make cream walls suddenly look dirty. The perception is also relative — the visual system tends to treat the brightest, least tinted thing in view as white and judge everything else against it.
That relativity is called colour constancy, and it is the reason a room painted entirely in cream simply reads as white until something truly white is brought into it.
White has a particular role in colour measurement: it is the reference. White balance in photography means telling the camera which part of the scene should be treated as neutral, and every colour in the image is then interpreted relative to that decision. Get it wrong and the whole photograph takes on a cast. In colour spaces such as CIELAB, a defined white point is what makes the numbers mean anything at all. So whether or not white is 'a colour', it is structurally special in a way that, say, teal is not.
Wavelength ranges and perceptual thresholds are given as approximations: both vary between individuals and with viewing conditions, and sources differ on exact boundaries. Where a figure is contested or unavailable, this page says so rather than picking one.