Achieving harmonious colour relationship for visual expression is of prime importance in art and design. Mixing pigment colours is often unwieldy, ambiguous, confusing and problematic. Therefore, as a panacea, this paper proposes a mathematical model that would serve heuristic, descriptive, diagnostic, prescriptive, and predictive functions. It used a triangulation of the Critical-Historical-Analytical Examination, Artistic Exploration and Content Analysis methods. The paper started with the need for model to satisfactorily resolve problems associated with mixing colours in harmony. Furthermore, the fundamental features of colour were highlighted. Moreover, it discussed the adoption and adaptation of the Colour Wheel into a set of alphanumeric codes and mathematical proposition: nH(I+V) =1, where n is a number of a code that represents a type of colour harmony, H:Hue, I: Intensity and V: Value. The alphanumeric codes: M(1), A(1,2,3), DC(1,7), SC(1,6,8), NC(1,6 or 8), TR(1,5,9), B(1,6,10) and TD(1,3,7,9 or 1,4,7,10) represent Monochromatic, Analogous, Direct Complementary, Split Complementary, Near Complementary, Triadic, Balance, and Tetrad schemes respectively. And the paper ended with suggestions that artists and designers should internalize and employ these codes and formula, which with practice, become intuitive and useful.Keywords: Colour Features, Colour Mixing, Harmonious Relationship, Mathematical Model, Art and Design