Don't get frustrated if you guess wrong. Technically there is no way to know the correct color for sure if you haven't seen the picture (or film) before, because there is never just one color possible. The colors that are in the film are there as a result of choices the makers made for several reasons. Great artists follow their intuition and still adhere to all the "rules" of composition and color theory without thinking about them. Still, the result can be analyzed and some of these reasons can be understood in the context of the film or its historical circumstances. Sometimes even the most unexpected colors look organic in context, but we wouldn't pick them if we had to guess from a gray scale image.
What I like about guessing colors is that it helps sharpen our preception in other films and even everyday life. It has been said that humans possess a poor color memory. I personally doubt that, it's just something that most of us don't cultivate. So if you have seen one of the pictures before and as a result you remember the correct color easily, it shows that you already have developed an eye for this. Sometimes there's even a narrative reason why you remember a color as in the example of Steal Wool where the suspending bridge is made out of fire crackers which are traditionally red in cartoons.
If I'd shown you this picture you would have probably all guessed right. By the way, red is already firmly established in the cartoon before (nose and attendance clock):
Today's image comes from Nina Paley's one-woman-show Sita Sings the Blues, a film whose visual appeal relies heavily on the lush vibrant colors. The following is from one of my favorite scenes. It is visualizing Annette Hanshaw's rendition of "Am I Blue".
What color is the priest wearing?
In case you are new, THIS is how it works.