推荐回答（4个）
这是因为大部分的植物都含有大量绿色的叶绿素和橙红色的胡萝卜素 ,帮助进行光合作用 ,制造养份。所以多数蔬果 ,都是黄绿色或橙红色的。其实 ,我们也有一些紫色的蔬果 ,例如蓝莓、茄子和紫葡萄。它们的果皮含有一些化学物质 ,叫做丹灵类（ "tannins",有坚固细胞壁的作用） ,这种物质与蔬果中的铁质结合后 ,便会变成深蓝色。但是这些蔬果也含有其他色素 ,所以显现出来的并非纯蓝色。此外 ,蔬果里的另一种化学物质花色（音代 ,"anthocyanin"） ,在不同的酸碱度呈现不同颜色 ,酸性的时候呈现红色 ,中性的时候呈现蓝色。大部分的水果都是酸性的 ,所以很少有蓝色的水果也不足为奇啊。
这是因为大部分的植物都含有大量绿色的叶绿素和橙红色的胡萝卜素 ,帮助进行光合作用 ,制造养份。所以多数蔬果 ,都是黄绿色或橙红色的。其实 ,我们也有一些紫色的蔬果 ,例如蓝莓、茄子和紫葡萄。它们的果皮含有一些化学物质 ,叫做丹灵类（ "tannins",有坚固细胞壁的作用） ,这种物质与蔬果中的铁质结合后 ,便会变成深蓝色。但是这些蔬果也含有其他色素 ,所以显现出来的并非纯蓝色。此外 ,蔬果里的另一种化学物质花色（音代 ,"anthocyanin"） ,在不同的酸碱度呈现不同颜色 ,酸性的时候呈现红色 ,中性的时候呈现蓝色。大部分的水果都是酸性的 ,所以很少有蓝色的水果也不足为奇啊。
另外更全面的回答可以去这看看
http://www.zhihu.com/question/30735969
有蓝色的水果和蔬菜。
蔬菜中的蓝菜苔（又叫红菜苔，但下锅煮熟时呈深蓝色），其他的还有：茄子、扁豆、甘蓝、紫菜、苋菜、茄子、山竹、洋葱、
水果：蓝莓、桑葚、葡萄。
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