Jonas loved all of the colors, but I would say his favorite color is red because it was the first color he saw.
In Jonas’s world, no one sees color. Sameness means a world without color, so everyone apparently sees in black and white and shades of grey.
When Jonas is playing catch with his friend Asher, he notices that the apple has changed.
Jonas had noticed, following the path of the apple through the air with his eyes, that the piece of fruit had—well, this was the part that he couldn't adequately understand—the apple had changed. (ch 3, p. 24)
When Jonas is selected Receiver of Memory, he is told that he has the Capacity to See Beyond. The Giver later tells him that this means he is beginning to see color.
"There were a lot of colors, and one of them was called red. That's the one you are starting to see."
In Jonas’s world, seeing color makes him unique. Jonas’s first memory is also of the color red, when he sees the red sled. This is another reason why he would prefer the color red.
Lowry, Lois (1993-04-26). The Giver (Newbery Medal Book) (p. 94). Houghton Mifflin Harcourt. Kindle Edition.
Jonas loved all colors as he lived in a world where there are no colors.
In a world where there are no colors, Jonas felt that all the colors were beautiful . I feel like though that his favorite color is red because it was the first ever color that he was able to see .