The only way to darken your skin color without using a tanning bed is to sun bath. Tanning is the only way in which you can darken the color of your skin - there is no other way to do so. Your skin color is determined by the amount of skin pigment it contains. This pigment is known as melanin, and the amount of melanin in your skin is primarily determined by your genes. Your skin can however produce more melanin, and this is determined by your exposure to sunlight. There is of course a limit to how much melanin your skin can produce, but when exposed to sunlight, certain cells in the skin tend to produce more melanin. In addition, existing melanin in the skin gets oxidized in the presence of sunlight, and therefore becomes darker. Tanning is the result of these two processes, and is the only way to darken skin color - and as mentioned before, it has its limits too. If you are Caucasian, your skin color will always be relatively light, no matter how much time you spend in the sun or in a tanning bed.
In addition, spending too much time in either place can have negative effects as well, the least of which is a sunburn. The amount of exposure that is required in order to develop a tan is sometimes quite dangerous. Long periods of sun exposure can damage the skin and cause a lot of problems, including serious conditions such as skin cancer. It is therefore advisable to avoid exposing yourself to UV light for too long or too often, whether this is from sunlight or from a tanning bed. If you must tan, you should do it for short periods of time; if this is done regularly, you will develop a tan without risking your health too much.
answered by M W