The bottom is called a hakama. Traditionally in kendo, the hakama is black or indigo blue for men and white for women. The gi is blue or white. Iaidoka sometimes wear all black or all white regardless of sex. Children's gi have a diamond-shaped line pattern on them.
Most people wear all blue.
A good quality Japanese gi is died with natural indigo, and so is the kendoka wearing it until the salt from sweat sets the dye. You can also cheat and wash a new gi in cold water and salt before wearing. Hakama and keiko-gi are robust versions of the formal samurai clothing of the 18th and 19th centuries. They are worn during sword practice, in preference to something like the clothes worn in karate, to emphasize the formality of occasion. Kendo or iaido training is meant to be more than just physical training, and the choice of clothes emphasises this.