the problem is the icon shown is not network-workgroup-symbolic but simple network-workgroup with colors.

What do you mean by this, what colors if the icon itself is monochrome?

network-workgroup is color, the network-workgroup-symbolic is monochrome.

icon-development-kit only has one icon, where is the second one with colors coming from?

From system resources, i guess. I don't even need the icons crate btw for both icons. I have the latest Fedora Workstation installed.

use gtk::prelude::{BoxExt, GtkWindowExt};
use relm4::{gtk, ComponentParts, ComponentSender, RelmApp, RelmWidgetExt, SimpleComponent};

struct AppModel {}

struct AppWidgets {
    image1: gtk::Image,
    image2: gtk::Image,
}

impl SimpleComponent for AppModel {
    type Input = ();
    type Output = ();
    type Init = ();
    type Root = gtk::Window;
    type Widgets = AppWidgets;

    fn init_root() -> Self::Root {
        gtk::Window::builder().title("Icons test").build()
    }

    /// Initialize the UI and model.
    fn init(
        _: Self::Init,
        window: Self::Root,
        _: ComponentSender<Self>,
    ) -> relm4::ComponentParts<Self> {
        let model = AppModel {};

        let vbox = gtk::Box::builder()
            .orientation(gtk::Orientation::Vertical)
            .spacing(5)
            .build();

        let image1 = gtk::Image::new();
        image1.set_margin_all(5);
        image1.set_icon_name(Some("network-workgroup"));

        let image2 = gtk::Image::new();
        image2.set_margin_all(5);
        image2.set_icon_name(Some("network-workgroup-symbolic"));

        window.set_child(Some(&vbox));
        vbox.set_margin_all(5);
        vbox.append(&image1);
        vbox.append(&image2);

        let widgets = AppWidgets { image1, image2 };

        ComponentParts { model, widgets }
    }
}

fn main() {
    let app = RelmApp::new("relm4.test.icons");
    app.run::<AppModel>(());
}

[package]
name = "test_gtk_icons"
version = "0.1.0"
edition = "2021"

[dependencies]
relm4 = "0.9.1"

Hm... I think that might be a system theme that you have installed, interesting.
Depending on use case you may or may not actually want that behavior 🤔

I haven't installed any theme, just default Fedora installation. Or what could I have installed?

Are you sure network-workgroup-symbolic comes from relm4-icons at all? There is a chance it comes from system icons instead and network-workgroup is what comes from an app, but some of its icons are overridden by one of the system themes. I use Ubuntu and there are some non-symbolic icons installed via dependencies like Humanity. Check in debugger what it loads and from where (Ctrl+Shift+d).

I have all these icons already installed in my system, they come with Fedora distribution. But in case I want my app icons look the same on other systems I need to override these icons resources with relm4-icons. And since the crate has only symbolic versions of icons, I need to have all icons names with the -symbolic suffix.

I see. Don't get me wrong, I do think it is a fair request, I'm not 100% sure why removing -symbolic was necessary in the first place, maybe it should be reversed. I also noticed some icons being re-styled on my systems, likely those that matched icons installed on the system.

What do you think is it a good idea to have only own icons resources and never allow use of system icons?

Hard to say. On one hand I'd like to control the way application looks, on the other hand I'm glad I'm able to install Numix icons pack and many of the apps I use look better. I guess it needs to be configurable 🤔

Also there could be a problem when there is no particular icon in a system.