When it comes to colors which are used on Holi, there is less awareness to avoid harmful chemical colors and to use herbal colors. The history of Holi goes back to thousands of year and it was already in vogue during the time of Lord Krishna. Lord Krishna only added to its glory by playing it with Radha Rani and other female cowherds of Barsana.
Hindus were playing Holi with red and green colors even when the red of Mercury Sulfate (HgSO4) and green of Copper Sulfate (Cu2SO4) were unheard of. Since Vedic time people were playing Holi with herbal colors. Most temples in Braj region use natural color to play Holi. Tesu is one of the most commonly used flowers to create natural color.
Tesu (टेसू) is the Kesar color flower of Palash(पलास) tree. The trees of Palash are easily found all over India and its parts are used in various products and herbal medicines. However it is popularly known for its flowers which are used to extract yellow color during Holi.
It is believed that Lord Krishna used to play Holi with Tesu flowers and hence most Krishna temples in Mathura and Vrindavan use them to play Holi. Tesu flowers are soaked in warm water one day before of Holi. On the next day the concoction is used to play Holi. Usually, the rose water, Chandan, Kesar and scents are added to the concoction.
There is custom to throw wet colors on devotees who visit temples on Holi. If you are blessed with that scented yellow color while you visit the temple then most probably it would be the concoction of Tesu.
Tesu flowers are easily available at various grocery shops which sell other herbs and spices. Tesu flowers cost around Rs. 50-70 per kilogram.