I wonder how many times in real life the bomb squad doesn't cut the right wire and they all get blown up? On screen they never do. Why?

Generally, the person disarming the bomb has more experience than the person making the bomb ... and most bombs are made to kill/injure innocent civilians, rather than someone with military training in disarming bombs.

I never understood the amateur bomb makers using different colored wire.  If I were buying supplies to make a bomb, I'd save money and only buy one color of wire.  Of course, it would make it harder to keep track of what was going where - but temporary tape would easily solve that problem.

Most of the terrorists' weapons are probably very basic, too. Identify the battery or the detonator(s), cut either of the wires singly, and it's inert. The real difficulties are with land-mines, and the explosives packs worn by suicide-bombers. 

I suspect the complexity of the bombs in films is made up for dramatic effect anyway. 

Most conventional military bombs and artillery shells have impact detonators, working rather as in small-arms rounds, not electrical circuits; though they still need enormous care, patience and skill to render safe. Unexploded explosives and gas shells are still being found on the Western Front 100 years later, because the ground became so churned up the ordnance simply buried itself in mud soft enough not to trigger the percussion-cap. 

An interviewee I recall on the radio news around the time of the big East-West disarming treaties, explained that the easy part is physically dismantling the nuclear weapons: "just a matter of screwdrivers and spanners". One supposes the tools are used by people who know what they are doing.

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The daftest fictional disarming I have watched was in a barmy sci-fi film I think called The Abyss. The bomb was nuclear, so a quite complicated weapon; but it was on the deep ocean floor where one would expect the pressure to have crushed the casing or at least overcome the weather-proofing seals; flooding it with salt water, shorting all the electrical parts, discharging the batteries and possibly dissolving the explosive charges that make the nuclear "fuel" ignite. But that would not be so dramatic, now would it... Our hero, equipped with world-saving wire-cutters, was carefully talked through it by his controller on the ship thousands of feet above - "Unscrew the cap... What colour wires can you see...?" Then "Cut the red one..., now the blue... " etc. I wondered if the colours would be clearly visible not only underwater, but illuminated only by torch-light in the pitch-blackness of the abyssal plain.