The Chocolate albatross (Appias lyncida) is a fairly common butterfly in Asia, which can be found from Sri Lanka to Borneo. There are a number of different subspecies across this vast range. I was very pleased – I am always happy to see a butterfly! – to come across this fine specimen in northern Thailand (Chiang Dao) where it was nectaring in some flower beds.
This individual was a male, and is, I think, Appias lyncida vasava, on account of having a rather thick brown band on the hindwing (uds), but I am not 100% sure! Yellow is often a warning (or aposematic) colour, but these particular butterflies are not poisonous.
This butterfly is rather variable in appearance. First of all the sexes look quite different; and then there are quite different forms at different times of the year: wet and dry season forms (together with some intermediate forms if that is not variable enough!). The forewings of the females are darker, particularly on the dorsal (upperside), as my not very good photograph (below) illustrates.
In the so-called, Wet season, the differences between the sexes becomes more pronounced, with the female being much darker: heavily marked with brownish-black markings on the dorsal surface (below, right hand side). These days, there are, I think (at least people tell me), less clear-cut differences between the seasons in countries like Thailand. Nevertheless, the wet season forms were beautifully illustrated by an British artist called John Nugent Fitch (below) in a book on the butterflies of India, by Frederic Moore, published in 1903-05. The darker markings on the wet season forms are thought to be to enable the butterflies to absorb more solar radiation, and thus heat up more quickly in inclement conditions!

Appias hippoides (=lyncida) from Frederic C. Moore (Plate 540, Vol. 6. Males on left, females on right. Top: Wet season brood. Bottom: Intermediate Wet Season brood.
Lepidoptera Indica.There is also a lovely photograph, on the Wikipedia entry for Appias lyncida, of a pair of Chocolate albatrosses (below).

Chocolate albatross (Appias lyncida) pair. Balakrishnan Valappil [CC BY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0)%5D

The Redbase Jezebel (Delias pasithoe) upperside. Hectonichus [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)%5D
Quite why we find brightly coloured creatures like this, so attractive, is another, deeper, question; but let’s leave theorizing aside for now, and I hope readers will just enjoy the spectacle in the following images!
References
Canfield, M. R., & Pierce, N. E. (2010). Facultative mimicry? The evolutionary significance of seasonal forms in several Indo-Australian butterflies in the family Pieridae. Tropical Lepidoptera Research, 20(1), 1-7.
Lepidoptera Indica. Volume 6, Plate 540. Lovell Reeve & Co, publishers https://www.biodiversitylibrary.org/item/103503#page/207/mode/1up

My soon to be published book! https://www.cabi.org/bookshop/book/9781789242638/