Colour pixels are different from grayscale pixels. Colour pixels are RGB, meaning they have three pieces of information associated with them, namely the Red, Green and Blue components. Grayscale pixels have one component, a gray tone derived from a graduate scale from black to white. A colour pixel is generally 24-bit (3 × 8-bit), and a gray pixel is just 8-bit. This basically means that a colour pixel has a triplet value comprised of 0..255 for each of red, green and blue components, whereas a grayscale pixel has a single values 0..255. The figure below compares a colour and grayscale pixel. The colour pixel has the R-G-B value 61-80-136.The grayscale pixel has the value 92.
It is easy to convert a pixel from colour to grayscale (like applying a monochrome filter in a digital camera). The easiest method is simply averaging the three values of R, G, and B. In the sample above, the grayscale pixel is actually the converted RGB: (61+80+136)/3 = 92.
Now colour images also contain regions that are gray in colour – these are 24-bit “gray” pixels, as opposed to 8-bit grayscale pixels. The example below shows a pixel in a grayscale image, and the corresponding “gray” pixel in the colour image. Grayscale pixels are pure shades of gray. Pure shades of gray in colour images are often represented with RGB all having the same value, e.g. R=137, G=137, B=137.
A pixel is an abstract, size-less thing. A pixels size is relative to the resolution of the physical device on which it is being viewed. The photosites on a camera sensor do have a set dimension, but once an image is acquired, and the signal are digitized, image pixels are size-less.
For example, let’s consider TVs, and in particular 4K Ultra HD TVs. A 43″ version of this TV might have a resolution of 3840×2160 pixels (w×h). The 75″ version of this TV has *exactly* the same number of pixels – about 8 million of them. What changes is the pixel size, but then so does the distance you should view the TV from. The iPhone 11 in comparison has a screen size of 1792×828. For example, the 43″ 4K TV has dimensions of roughly 37″×20.8″, which means that the size of a pixel is 0.24mm. A 75″ 4K TV would have a pixel size of 0.41mm. An Apple Macbook Air with a 13.3″ screen (2560×1600 pixels) has a pixel size of 0.11mm.
As an example consider the image below. Two sizes of pixels are shown, to represent different resolutions on two different physical devices. The content of the pixel doesn’t change, it just adapts to fill the physical pixels on the device.
Likely more important than the size of pixels is how many of them there are, so a better measure is PPI, or pixels-per-inch. The iPhone 11 has 326ppi, a typical 43″ TV has 102ppi, and the 75″ TV has 59ppi.