That's normal. The Denon has a noise block screen. This happens also after watching a VCR tape, when there's noise.
First of all, the Denon offers higher grade video circuits than any receiver I've had. Denon uses "MECHANICAL RELAYS" instead of the inferior, bandwidth limiting Integrated Circuits(ICs) that most other receivers use. The clicking is the relays.
The problem your having is most likely because of your cable box. I use Directv and do not have that problem. What your describing, happens when the noise circuit tries to put up a blue screen when video noise is detected. The clicking is the relay that does this. Your cable box must be very slow in changing channels, therefor triggering Denon's noise blocking circuit and causing the relay to click in, to display the blue screen. The green and purple lines are noise that's created by the relays opening and closing, using the component output. With the S-Video output, the color is then grey instead.
This is completely normal. With other components I have, when they use their noise blocking circuit(by putting up a blue screen also), initially there is also a glitch of noise and these units use IC circuits.
The Denon has different modes that 'may' help with this. Try the ON SCREEN MODE 2 instead, this may help. Blue screen are displayed to block noise and allow on-screen menu text to be displayed correctly. Noise will not allow on-screen text to be displayed correctly and is why components offer the blue screen noise blocker. Mode 2 most likely will correct your specific problem.
BTW, receivers that don't offer this blue screen can only have their on-screen menus read while using a stable video signal. Denon gives the On Screen mode to allow you more flexibility.
Unfortunately Denon should had mentioned if a problem as yours, is happening, to use Mode 2.
Some of my VCRs put up a blue screen without the option to disable it. Denon does give you the option to disable it. It's frustrating because on certain parts of movies where this is a bad spot on the tape, the blue screen comes on and I miss that entire scene. Very frustrating.
Good luck.
