Dark red, green, orange, orange-red. in the apple tree there are 2 reasons 
1)it absorbs the rest of the colours and reflects the red colour that we can see through our eyes
2)he apple fruit cortex is usually colourless, although germplasm does exist where the cortex is highly pigmented due to the accumulation of either anthocyanins or carotenoids
i mean why is the color red only not any other?
nice question
hope this helps
Apple red (occasionally known as apple-red) is the name code used by manufacturing companies to define a shade of red similar to the red sugar coating on  apples. The typical method for producing a candy apple finish is to apply a metallic base-coat, followed by a translucent color coat. A final clear coat adds additional gloss.