Like for any flag colors. There are always several interpretations, beside the "official" ones (red is blood, and white is spirit). Even if the berani/suci is an interpretation a posteriori, it's symbolically important for people.
But the true story is that the colors of the Indonesian flag are taken from this flag:
https://en.wikipedia.org/wiki/Majapahit, a buddhist empire.
The Indonesian national flag
"Sang Merah Putih" ("Red and White") or sometimes called "Dwiwarna" ("The bicolor"), derived from the Majapahit royal colour. The Indonesian Navy flag of red and white stripes also has a Majapahit origin. The Indonesian national motto, "Bhinneka Tunggal Ika", is a quotation from an Old Javanese poem "Kakawin Sutasoma", written by a Majapahit poet, Mpu Tantular.[8
Kayanya ga selalu gitu, deh. Maksud gue, kedua warna itu punya beberapa makna yang lain. Gue juga pernah baca kalo merah melambangkan darah dan putih melambangkan tulang.
It is not always like that, I think. I mean, the colors have some other meanings, though. I also have read that Red represents Blood and White represents Bones.