A system and a method of adjusting colors of print data are provided. The system includes a printer driver converting a document transmitted from an application program into print data suitable for a printer and transmitting the print data to the printer. A soft proofing unit applies a soft proofing process to the print data converted by the printer driver. A display unit displays the print data obtained through the soft proofing process. A color lookup table (CLUT) supplies a color adjustment value to the print data when the print data is processed in the soft proofing unit. A system and a method of adjusting colors of print data is also provided to allow a user to accurately predict and adjust the colors of a printout, thereby obtaining the printout having the desired colors. Additionally, a method is provided of adjusting colors of print data to modify and apply a color lookup table (CLUT) for converting RGB image data into CMYK image data in a print driver by performing a soft proofing process and using a gradation chart for a preference of a user, thereby increasing user satisfaction of colors.