Caterpillar was the pioneer in adopting yellow as the colour for construction machinery. In the early 20th century, their equipment was grey, influenced by military usage. However, it was recognized that for increased safety on roads and at construction sites, these vehicles needed to sport a high-visibility colour. Thus, in 1931, the company opted for a yellow hue. An unintended consequence was branding: The yellow machines drew attention, stood out, and helped people remember the manufacturer. So, it was not long before other companies followed suit.

Source: Heavy Equipment Guide