Synthetic wigs are a much more affordable alternative to human hair wigs. Unfortunately, because of this they don't last as long as human hair wigs and are also more limiting as far as shape and style options. They also require extra care to keep them looking clean and smelling fresh. However, with proper maintenance and use, synthetic wigs are a very practical and affordable option---whether you need a wig for personal or professional use. Unfortunately, they cannot be lightened.
Human hair wigs can occasional be dyed lighter, although this is a risky undertaking that can prove difficult. However, synthetic hair wigs are made of a nylon that has the color built into the pigment and therefore doesn't absorb color like real hair does. Because of this, there is no way to lighten a synthetic wig. If you want to buy a wig that you can lighten, you will have to buy a human hair wig which generally cost upwards of $200. Synthetic wigs, on the other hand, are often almost 1/10 of the price of a human hair wig.
Although you can't lighten a synthetic hair wig, you can darken it. This is because the plastic hair fibers of synthetic wigs will bind with certain stains, such as fabric dyes. To stain a synthetic wig darker, mix about four ounces of fabric dye with one gallon of water and soak the wig until it attains the desired color.
If you try and stain a synthetic wig lighter, it will not work and the original, darker color will prevail over any lighter stain or dye.
Buy a lighter colored wig if you need one. The nice thing about synthetic hair wigs is that they are relatively affordable, which means it's perfectly reasonable to own a variety of synthetic wigs for different occasions. If you find yourself needing a lighter wig, the easiest thing to do is to go and buy one. Buying a human hair wig and then dyeing it will be much more expensive and time-consuming.