Secondary colors are colors that can be made by mixing primary colors together. Orange, green and purple are all secondary colors because they are made by mixing red, yellow and blue (the primary colors) together. For example, yellow and blue make green. Red and yellow make orange and red and blue make purple. It is also arguable that pink is a secondary color because it is created by mixing red with white. Although white is an achromatic color, not a primary, it is still natural (and not creatable via mixing other shades together) and, therefore, anything it creates via mixing is a secondary shade.
Red, yellow, blue, black and white are all unique and original colors because they cannot be created by combining two other colors together. However, these five colors can be mixed to create other colors. Secondary colors like green, orange and purple can also have a wide range of shades and hues. Green is particularly versatile since it includes shades that are very light and cheerful, such as lime green, while other forms of green, like forest green, are so dark that they are almost black.
Secondary colors generally mix with black and white—and occasionally other primary or secondary colors—to make up different hues. All of these hues are present on the color wheel and offer people a lot of options for color shades across numerous gradients.