Imagine that number one is red and number two is yellow, you can find between one and two : "1.2, 1.21, 1.22, 1.3, 1.4, 1.45....". Same with colors, there are unlimited amount of colors between every two colors the thing that gives us gradation.

TL;DR in Vietnamese:
Vì số là vô hạn, số lượng màu sắc cũng vô hạn.