I can see a few problems with this as you would need 2 or 3 loads and you could have a variation of colour with each. I am always dying articles and have had some horrendous results.Morrisons sell fabric dyes as do local hardware stores.
Was going to say the same thing. If it came out really awfully would it be a catastrophe? Because the chances of you getting a good outcome are slim especially if the fabric is a mixed fibre with any synthetic in it.You will also have to thoroughly clean out the washing machine afterwards.
If they have just faded and you are just hoping to even out the colour you might get away with it but you are talking about a minimum of 4 loads and two boxes of dye and half a kilo of salt in each load with a lot of dye going through the machine... I've done it with the old dylon but the new one I find is less reliable I think due to the lower temp the other problem is you will get dye rubbing onto clothes for some time afterwards.