In plasma 4 GTK applications used Qt colours, in plasma 5 GTK applications ignore Qt colours and use something else. Looks like GTK colours are hardcoded into GTK schemes? How can I make GTK applications use the same colours as Qt? Or at least change GTK colours - edit GTK themes or something?

I don't fully understand the problem...
My GTK applications use the same colours
I'm assuming that your widget styling has been configured so that any GTK apps look like your DE theme
I've yet to see this not working...
unless it's the case that I've misunderstood you issue!

I think it partially depends on the application, in particular whether the application uses gtk2 or 3.

I'm pretty sure the theme engine used for gtk2 themes should update to use the KDE color scheme.

Older gtk3 themes may have been able to support this, but recent versions generally hard-code the colors in the theme itself. IMHO, it's an obnoxious trend that grants the theme designers total control over the color scheme used in the theme (denying those of us who are used to selecting our own color schemes the privilege to do so, except insofar as the designer provides color variations (because those are the only colors for which that theme will look right)).

While they were developing the GTK version of the Breeze theme, there was an experimental branch which included a script to update the colors of the various assets used in the theme to match your current KDE color scheme. Unfortunately it was abandoned before they copied the theme into the KDE repos.

At present, it looks like the only options for Gtk3 are to use Breeze or Breeze-dark, or some other gtk3 them for which you are subject to the designer's taste in colors.

Yes, airdrik, you are quite correct - it is GTK3 applications which are affected. Are there any tools for modifying GTK themes?

Besides standard text and image editors, I'm not aware of any (not that that means they don't exist, only that I haven't bothered to look). However, if I understand correctly, a gtk3 theme is just composed of a collection of css and image files or something along those lines; so you can browse through the files in the theme and make your own changes to them if you wanted. I believe you can also use tools like imagemagick to change the colors of the theme images.

The named-colors branch of the original gnome-breeze repo has a script that was designed to update the resources in that theme to match your KDE color scheme, which could probably be adjusted to work with other themes.