A Percent column type is used to enter a percentage value like the percentage of project completion. When you enter the value in the cell, you will be able to see the value as well as a line representation of the percentage value.
The color of the line depends upon the value and helps in better visualization. By default for values between 0-39 the color of the line is red, for values between 40-79 the color of the line is blue and for values above 79 the color of the line is green.
These values can be changed by clicking the Limit option and then defining the Minimum and Maximum values.
All the values above the maximum value will be represented in green. The values in between the maximum and minimum values will be represented in blue and the values less than the minimum value will be represented in red.