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 Vietnamese:
Tìm dầu gội tím có màu và kết cấu đậm đặc. Mua dầu gội tím đậm màu hơn cho mái tóc màu xám, màu bạc hoặc bạch kim. Chọn dầu gội tím nhạt màu hơn nếu bạn có tóc màu vàng hoe. Tránh dùng dầu gội tím nếu bạn có tóc sẫm màu.