The discussion revolves around using the RectangleChart function in Mathematica to color rectangles based on a third variable, z, which is a numerical value scaled between 0 and 1. Initially, the user struggled to apply color using ChartStyle or Style options. A solution was proposed to color individual rectangles by using the Style function with specific color names or by setting ChartStyle with a list of colors. However, the user clarified that their z values were numerical and not color names, prompting a request for a method to convert these numbers into colors. The solution provided involved using the Map function with ColorData to generate a list of RGBColor entries based on the numerical values, allowing for the effective application of color gradients in the chart. The discussion highlights the importance of color schemes in data visualization and offers practical coding solutions for users working with Mathematica.