Try reloading the page to reset all the variables. By changing $\omega_0$ to $3 \cdot 10^{15} \text{ rad/s}$, and increasing it slowly from there, you may notice that the color of the light emitted in the direction of motion of the emitter changes, from green to blue. But the light emitted in the other direction does not really change that much in color; it remains red for a while. But the Doppler effect affects both directions by the same factor... The trick is, the visible spectrum has almost twice as much red in it as blue or green.

Try changing $v_1$ and $v_2$ to their maximum values and slowly increase $\omega_0$ too. After a while, the light registered by the observer in front of the emitter (to the right side of the screen) turns instantly from violet to pink. This is just a visual trick. After that nice color of violet, we slowly move outside the visible spectrum. The pink color is supposed to represent ultraviolet light.