If your hair is approx the same colour as Sienna Miller or Naomi Watts or even Lauren Laverne...

[civilfawlty]

... and you dye it at home, please would you let me know what dye (colour and brand) you use. Also helpful to know what your original colour is and which celeb it's most like! I'm trying not to make any more expensive mistakes... at this rate, it would be cheaper to go to a salon

Thank you!

It all depends on your base colour. One home hair dye will come out very differently on two people iyswim.
Tell me what your starting colour is and whether it has any gingery or red tones and I will try to recommend something.

Amazing. Thank you.

My natural colour is mousy brown and a fair few greys littered through it.
Until recently, I've been using the 6-8 wash colours to add a little warmth. But I've decided I want to be definitely blonde. I've used two dyes recently, and realised that the "golden" blondes are too yellow for my skin tone.
Does any of that make sense?

Yes that makes sense.
I have dark blonde mousey coloured hair and to get it to a light blonde I use L'Oreal Excellence pure blonde 01 in Ultra light natural blonde. This is a high lift colour so will lighten your hair to the light blonde you want without brassy tones.

You will have to be brave about this because it's quite a change!
Light blonde hair is high maintenance too. You'll need to retouch your roots every few weeks and invest in decent conditioners and oil treatments to keep it in good condition.
My tip is to use a purple toning shampoo when you wash it. Boots Lee Stafford bleach blonde shampoo is brilliant at neutralising yellow undertones.

Good luck, maybe do a strand test if you're nervous.

You star. Thank you SO much. My supplementary questions are:

1. If I dyed my hair yesterday, can I dye it again today?
2. If I mix the dye up, can I do a patch test and then, if I like it, can I use the rest of the dye after 30 mins, or will it have stopped working?

You can dye it again if it's in good condition with the no signs of chemical damage.
The dyes contain bleach so you have to be a bit careful. Use lots of the conditioner included afterwards. The L'Oreal one also has a tube of serum included that you can rub into the ends of your hair to protect them from damage before you start dying it so I recommend using that.

If you've mixed up the dye, you have to use it straight away, otherwise it will start developing in the bottle. For a test, pour a tiny amount from each bottle (equal parts) onto a non metallic container, e.g. a saucer or plastic lid, and mix together. Screw the lids back firmly on each bottle and you can use them again later.

I sometimes pour out half of each bottle and mix them to do my roots this way as I don't need a whole bottle. As long as they're not mixed, they will keep.

Here is how mine comes out. Sorry not a very good pic, was trying not to get my face in it.

That's very pretty. Thank you so much for sharing.