1. An image forming apparatus comprising: four first imaging units that use standard toners for colors of yellow, magenta, cyan, and black, respectively; a second imaging unit that uses a toner having a particle size larger than a largest one of particle sizes of three of the four standard toners that have colors of yellow, magenta, and cyan, wherein the second imaging unit uses a toner for at least one of the colors of green, orange and violet; a third imaging unit that uses a toner having a particle size smaller than a smallest one of particle sizes of the four standard toners; and a transfer unit that transfers first images formed by the respective first imaging units, a second image formed by the second imaging unit, and a third image formed by the third imaging unit to a recording medium such that the second image is provided on the recording medium, the first images are provided on the second image, and the third image is provided on the first images.