The speeds of red light and yellow light are exactly equal

In vacuum, all electromagnetic waves travel with the same speed $c = 3 \times 10^8 \text{ m/s}$, regardless of their wavelength. Thus, the speeds of red and yellow light are exactly equal in vacuum.

In a material medium like air, the speed of light is given by $v = \dfrac{c}{\mu}$, where $\mu$ is the refractive index of the medium. According to Cauchy's relation, the refractive index depends on the wavelength as $\mu = A + \dfrac{B}{\lambda^2}$. Since the wavelength of red light is greater than that of yellow light ($\lambda_{\text{red}} > \lambda_{\text{yellow}}$), the refractive index of air for red light is slightly less than that for yellow light ($\mu_{\text{red}} < \mu_{\text{yellow}}$).

Therefore, the speed of red light in air is slightly greater than the speed of yellow light ($v_{\text{red}} > v_{\text{yellow}}$). The speeds are exactly equal in vacuum but not in air.