The phenomenon you're describing, where colors of a continuous spectrum appear to blend or diffuse into each other despite having different wavelengths and frequencies, is known as color mixing or color blending. It occurs due to the way our visual system processes and interprets the different wavelengths of light.
Our perception of color is based on the response of specialized cells in our eyes called cones. Cones are sensitive to different ranges of wavelengths and provide information about the intensity of light at those wavelengths. The three types of cones in our eyes are most sensitive to short (blue), medium (green), and long (red) wavelengths.
When multiple colors of light enter our eyes simultaneously, each color stimulates the corresponding cone cells to different degrees. For example, if both red and green light are present, the red light will primarily activate the long-wavelength-sensitive cones, and the green light will mainly activate the medium-wavelength-sensitive cones.
The brain processes the signals from these cone cells and integrates the information to create our perception of color. When the responses of multiple cone types overlap or are activated to varying degrees, our brain interprets the resulting mixture of signals as a blended color.
For instance, if we mix red and green light, our brain interprets the overlapping signals from the long-wavelength-sensitive and medium-wavelength-sensitive cones as a perception of yellow. Similarly, when all three types of cones are stimulated equally by a mixture of red, green, and blue light, we perceive it as white.
So, even though different colors have different wavelengths and frequencies, our visual system processes the combined signals from multiple cones to create the perception of blended colors, resulting in the diffusion or mixing of colors in our visual experience.