The color of your eye is determined by pigmentation of the iris cells by varying amounts of melanin.

The iris is sandwiched between the cornea and the lens at the front of the eye, and surrounds the pupil, which expands and contracts to adjust the amount of light let in. There are two areas of the iris which may vary in pigmentation, the front and back surface. Generally the back has about the same amount of pigment in everyone, but the front varies a lot. Heavy pigment on the front causes more light absorption and browner eyes, less pigmentation causes increased reflectance and more blue or gray colors.

The more the eye reflects, the more likely it is that colors around you will affect the color of your eye.

People with less-pigmented irises tend to experience a lot of daily eye color variation depending on the color of the light reflecting off of the iris. Your shirt color, the angle of the sun, the cloud cover, the presence of water...all of these things can cause your eye color to change more or less depending on how heavily pigmented your eyes are.

As we age, our eyes tend to increase their melanin production, and stress, medications, sickness, or eye problems can also cause changes over time.

The quick answer is that yours eyes don't change, but the way we see your eyes does. I said it was quick, not quick to understand. Here's what's happening. When we see something, we are really seeing light that comes from some source (a lamp, the sun, etc.) then bounces off an object and into our own eyes. There are some great tutorials on light and color at:
light and color.

Just click on a topic and an animation will be activated.

The angle we observe an object from can change the apparent color of an object. Take a glossy photograph or magazine picture and look at it from various angles. The picture doesn't change, but the way we see it does. So does the light source. Look at the ocean on different days and from different angles (on the beach, from the pier, from the hills) and you will see a similar effect.

When different amounts and types of light (fluorescent, sunlight, etc.) hit your eyes from different angles, or we look at your eyes from different angles, they will seem to be different colors. When your pupil (the hole in the middle of your eye) is more dilated (open) or constricted (closed), the color will also seem to change. Imagine you stretch a balloon out; the color will lighten as the material stretches. Dilation or constriction of your pupil will also change light angles.

Your eyes may seem to change more than your friends' eyes do if you have different pigments (colors) in your iris (the colored part of your eye).

By the time you understand why the color of your eyes seems to change, you will have learned a lot about both color and how your eyes work.