Most homes are painted three different color groups, with the siding one color, the trim a second color, and doors and shutters a third color. There are exceptions, of course, especially for historic homes such as Victorians, which have their own color conventions. Bright colors are often used for trim, doors, and shutters, so repainting your front door red sounds like a good idea. Painting the garage door red, however, will draw attention to it at the expense of other, probably more interesting, aspects of your home. It is generally a better idea to paint your garage door the same color as your siding or leave it white.
If the garage and front doors both face the street, consider painting them the same color to unify your exterior color scheme. Also, remember to view your proposed color changes in the brightest sun, when colors really stand out, and then how they will look at just before sunset. These changes in light will affect color and contrast ratios. Red could go from fire alarm to mud.