A high-quality purple shampoo should be opaque, not transparent, in color. If you can, squeeze a small amount of the purple shampoo onto your finger before buying it to make sure it has a solid color.  Great options include Matrix So Silver purple shampoo and Paul Mitchell Platinum Blonde purple shampoo. You can find purple shampoo online, at local beauty supply stores, or at a salon that sells retail products. However, you may want to call the salon first to make sure they have it in stock. Darker purple formulas, some of which verge on indigo or blue in hue, work best on platinum, gray, or light blonde hair. Shy away from bright purple or violet shampoos and look for a dark shampoo made specifically for pale hair. Blonde hair needs less purple coloring to remove brassiness from its tone. Avoid inky violet shampoos and go for a brighter color to prevent oversaturating your hair. The lighter the color, the less brassiness it will absorb from your hair. Keep this in mind when choosing the right purple shampoo for you. Purple shampoo is ideal if you want to turn blonde or silver hair from a brassy to a brighter, more neutralized color. It is not as effective on brunette or black hair. If you have darker hair, try another shampoo treatment instead.

TL;DR in Chinese:
找一款粘稠的紫色洗发水。 灰色、银色、银灰色的头发可以选购深紫色洗发水。 如果你的发色是金色，选择亮紫色的洗发水。 如果你的发色很深，不要用紫色洗发水。