I guessed red hammer as well. I think the theory behind this is what we call in linguistics salience, a rather vague concept, but useful in instances like these: red is the most "salient" color because it sticks out more than any other. Red grabs your attention more than the others; that's why they use it for many kinds of warning signs, stop signs, etc. In linguistics, we also know that red is the color a language will have a word for if it has no other color words. For example, there are languages with only words for "black" and "white" (but they're actually broader in definition--usually red, orange and yellow are called "white" and green, blue and purple are called "black"), and there are languages with words for "black", "white" and "red", but no language with only "black", "white" and some other color, suggesting that red is somehow more different from the other colors, or more important, i.e. it is more salient. So when someone tells you to immediately think of a color, you're more likely to pick a particularly salient color like red. It is interesting that most of the answers here are among the four most salient colors according to what colors languages have colors for. If a language has words for "black", "white", and four other color words, it is overwhelmingly likely that those colors are "red", "yellow", "green" and "blue" (think about it -- that would be a lot more useful than, say, "orange", "purple", "brown" and "pink" for color words).

I'm assuming a similar thing applies to tools. The first tool to pop into your head is likely to be one that is frequently used -- a hammer, screwdriver, handsaw, or something, not more specialized tools like a vise or a power sander.