Continous variation is caused by combined effect of many genes and environmental factors. This is also known as polygeneic inheritance where single character is controlled by one or more gene such as skin color, height, weight, etc.
As height or length ranges from from shortest to longest any height is possible between these two extremes. So it is continuous variation.
Kernel colour in wheat is also an example of polygenic inheritance.
The two genes control the kernel colour of wheat each with two alleles, the one with the red kernel colour was dominant to the white.
The human skin colour is also polygenic trait because even though they donot seem to be affected by the environment, they are definitely polygenic traits and show a gradation, so they are definitely continuous traits.