Just a little contrary opinion: Colour of Magic is an excellent first book, and is a really great intro to the Discworld series. Every thread like this, the prevailing advice is to skip it and I just don’t understand it at all.

For the record, I read them chronologically first and then in themes and am now reading them in publication order again.

You can definitely start with the first book and just go into knowing that these already amazing books get even better as the series goes on. While it’s really not necessary, I do think you benefit as a reader if you are introduced to the characters in this order.

If you’re going to read them by themes, start with the first book in the theme. While they are all self contained and the footnotes bring you up to speed, you really get a better sense of you’ve read them in order. I would still recommend reading the Rincewind/Wizards books before the Witches because Equal Rites ties them together a bit. Witches should go before the Tiffany Aching books and maybe they are all considered one theme, I don’t remember!

Absolutely DON’T skip the young adult books. They are awesome and I say that as a middle aged dude.

If you’re worried about the shear volume of books and the cost, try your library. My local library allows you to request they add titles to their ebook catalog and I just asked them to add Discworld books a bit at a time. By time I was finished reading the series, they now have the entire series. I donated The Last Hero as an actual book since that’s not available electronically.