Kikki's Workshop Q&A: Let's ask Kikki ! Kikki's Workshop
Why are most construction equipment yellow ?
Probably because we associate yellow with "warning" or "caution". (Yellow cards are also used in soccer games.) Both people who operate construction equipment, and people who are around them should be very careful, and maybe that's why yellow is the most common color for them.

However, as a matter of fact, there are many more colors used for construction equipment other than yellow.
*Some small shovel cars have a color like this.