Answer by Balaji Viswanathan:
Hindu saints have always been amazed by nature. One of the biggest components of Vedas is the "Aranyakas" – experiments with nature. Two of the most impressive things that Hindu saints found were – Sandhya (time of sunset/sunrise) and Agni (fire). Big chunk of Vedas are about extolling the virtues of the Sun and Fire.
Hindus thought that the fire symbolized everything about us. The fire destroys everything it comes across into a lump of ash – whether it is your precious rug or an inexpensive cloth, just as a king eventually meets the same end as the poorest pauper. The rich burns as well as the poor.
Fire is a symbol of sacrifice and renouncing ego. During Yajna (fire worship), we offer plenty of dear things to the fire as an act of renunciation.
The three predominant colors of sunset and agni are yellow, orange/saffron and red. These three thus became the holiest colors of Hinduism. The bottom of the fire is usually red (where the coal is red hot), the middle is yellow and at the top it is orange (Fig 2). The same usually gets played in how we wear these holy colors.
Red is usually the color of worship – women wear kumkum (processed turmeric powder) and red sari during weddings/holy festivals (Fig 3). Yellow is usually worn by priests who are not monks (Fig 4). Saffron robe is usually reserved for the highest – those who have renounced everything (Fig 5).
Sandhya — figure 1
Agni — Figure 2
Red saree of the weddings — Figure 3
Yellow Robe of priests — Figure 4
Saffron Robe – Figure 5