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powder is only bad for colours if you use a powder that isnt designed for colours iyswim. Normal bio and non-bio powders contain bleach whereas powders for colours dont. I dont like liquids as they make the drum slimy and cause marks on the clothes.
Sorry I only use Persil non bio powder (tried lots of others but DH is allergic) but only a small scoop which always seems enough to get rid of the general dirt, specific marks & stains get stain remover. Colours all come out bright (except DSs rubbish school polo shirts) I use less than it says on the box though so they last a lot longer.
What washing powder/tablets/liqitabs do you use/recommend for coloureds. I have been using Ariel and Persil but they are so expensive and I don't always use fabric conditioner for school trousers and towels etc so I don't want to use the ones with it built in IYKWIM?