Single pixels

Individual pixels of white, red, green, blue, yellow, cyan/aqua, magenta/fuchsia
on a field of black in an uncompressed PNG file.
Use a jeweler's loupe to resolve the R G B components of the pixel triad of your monitor.
Set screen resolution to native resolution mode to avoid interpolation effects.
Ensure image is at its actual native size of 218x130.

"Blow-up" of the image's colored pixels:

Each pixel is a "triad" of three R G B components (subpixels),
roughly corresponding to the long (reddish), medium (greenish), and short (bluish)
wavelength cone receptor cells (6M of them vs 92M rod cells) of our primate's trichromatic eye.
The subpixels are too small to be seen (resolved) by human eye (acuity of ~1 arcminute).
Each component is represented by one byte (8 bits) indicating the intensity/brightness of that color (0 is off, 255 is full intensity).
Each pixel is three bytes (24 bits), so it can be any one of 2²⁴ = 16M colors.

100x100 image BMP (3B/pixel) an uncompressed image file format:
File size: 30,054 bytes

100x100 image PNG (a compressed format):
File size: 367 bytes      File size: 236 bytes

There is no yellow light here:

The screen is emitting only red and green light at each pixel.
Your brain, i.e. your mind, is creating the "yellow".

Computer screen is typically one of these (or larger):

width×height = the number of pixels.

wikipedia: More screen resolutions diagram

100x100 white and black pixels: