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I was experimenting with the bulb mode of my Canon EOS 600D camera. I realised, in the image there were these red, blue and white pixels that are also seen in the screenshot. What is the reason that I am seeing these pixels?

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These are so called hot or dead pixels, small sensor defects that are always there and are exacerbated by long exposures of mostly dark scenes.

These are the reason that there are "long exposure noise reduction" options on most cameras, these take a second exposure of equal length with the shutter closed, and look for pixels that aren't black even though they are supposed to be black - then make them black in the combined image.

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  • \$\begingroup\$ Sensor defects that are always there are called "Stuck" or "dead" pixels. "Hot" pixels are defects that only appear when the temperature of the sensor is raised, such as occurs during long exposures. \$\endgroup\$
    – Michael C
    Commented Oct 3, 2019 at 4:53

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