 In addition, given that humans can perceive over 2.8 million different hues, it has been suggested that the number of possible color combinations is virtually infinite thereby implying that predictive color harmony formulae are fundamentally unsound.Despite this, many color theorists have devised formulae, principles or guidelines for color combination with the aim being to predict or specify positive aesthetic response or "color harmony".Color wheel models have often been used as a basis for color combination principles or guidelines and for defining relationships between colors.