No, that’s very bad advice.
You should always match the pre-coloured extensions to the hair colour BEFORE fitting.
If you use any chemical colour treatments on hair extensions, you instantly invalidate any guarantee you might have with the supplier. You even need to be careful using purple shampoo.
Extension hair is sourced from hundreds of heads so the packs of hair that you buy is treated with very strong chemicals to strip the hair before being batch coloured and coated with silicone based products to ensure the colour lasts and that they feel soft after being sold.
Any attempts to colour them will likely result in patchy and uneven colour and they’ll lose their elasticity and smoothness.
Are you the hairdresser being asked to colour them?