The overall vividness of a color image is an obvious visual feature, but it is not easy to measure it quantitatively. The measurement involves the expression of the vividness of a single color and the quantitative description of a large sample of the vividness of each pixel in a color image. This article proposes a method to roughly measure the overall vividness of color images. The theoretically applicable color model for this method is the RGB color model. In this color model, the vividness of a single color is defined as the distance between the color point and the grey diagonal of the color cube. When calculating the overall vividness of a color image, first collect the vividness of each pixel in the image to obtain a distribution function, then use a suitable monotonic increasing function as the weight to integrate the distribution function and define the integration result as the overall vividness of the color image. In actual processing, integration is performed in a discrete sense. According to the conversions between the RGB color model and other color models, the method of overall vividness measurement can be extended to some commonly used user-oriented color models, including HSV and HSL. This article focuses on the theoretical method of characterizing the overall vividness, rather than finding an absolute formula to calculate it. In the experiment, this article uses the sRGB color model to collect data and uses a monotonically increasing linear function as the weight function to calculate the overall vividness of an image.