Add project files.

.idea/.idea.ColorPicker/.idea/.gitignore

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+# Default ignored files
+/shelf/
+/workspace.xml
+# Rider ignored files
+/contentModel.xml
+/.idea.ColorPicker.iml
+/modules.xml
+/projectSettingsUpdater.xml
+# Editor-based HTTP Client requests
+/httpRequests/
+# Datasource local storage ignored files
+/dataSources/
+/dataSources.local.xml

.idea/.idea.ColorPicker/.idea/encodings.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="Encoding" addBOMForNewFiles="with BOM under Windows, with no BOM otherwise" />
+</project>

.idea/.idea.ColorPicker/.idea/indexLayout.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="UserContentModel">
+    <attachedFolders />
+    <explicitIncludes />
+    <explicitExcludes />
+  </component>
+</project>

App.xaml

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+<Application x:Class="ColorPicker.App"
+             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:local="clr-namespace:ColorPicker"
+             StartupUri="MainWindow.xaml">
+    <Application.Resources>
+         
+    </Application.Resources>
+</Application>

App.xaml.cs

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+using System.Windows;
+
+namespace ColorPicker;
+
+public partial class App : Application {
+}

AssemblyInfo.cs

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+using System.Windows;
+
+[assembly: ThemeInfo(
+    ResourceDictionaryLocation.None, //where theme specific resource dictionaries are located
+                                     //(used if a resource is not found in the page,
+                                     // or application resource dictionaries)
+    ResourceDictionaryLocation.SourceAssembly //where the generic resource dictionary is located
+                                              //(used if a resource is not found in the page,
+                                              // app, or any theme specific resource dictionaries)
+)]

ColorPicker.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>WinExe</OutputType>
+    <TargetFramework>net7.0-windows</TargetFramework>
+    <Nullable>enable</Nullable>
+    <UseWPF>true</UseWPF>
+  </PropertyGroup>
+
+	<ItemGroup>
+		<Folder Include="Models\" />
+		<Folder Include="Views\" />
+		<Folder Include="ViewModels\" />
+	</ItemGroup>
+
+  <ItemGroup>
+    <PackageReference Include="CommunityToolkit.Mvvm" Version="8.1.0" />
+  </ItemGroup>
+
+</Project>

ColorPicker.sln

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+
+Microsoft Visual Studio Solution File, Format Version 12.00
+# Visual Studio Version 17
+VisualStudioVersion = 17.5.33424.131
+MinimumVisualStudioVersion = 10.0.40219.1
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "ColorPicker", "ColorPicker.csproj", "{527B253E-F795-40CE-9E4F-030717FB109B}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Release|Any CPU = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{527B253E-F795-40CE-9E4F-030717FB109B}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{527B253E-F795-40CE-9E4F-030717FB109B}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{527B253E-F795-40CE-9E4F-030717FB109B}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{527B253E-F795-40CE-9E4F-030717FB109B}.Release|Any CPU.Build.0 = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(SolutionProperties) = preSolution
+		HideSolutionNode = FALSE
+	EndGlobalSection
+	GlobalSection(ExtensibilityGlobals) = postSolution
+		SolutionGuid = {C45379AD-6848-4D51-9A8D-2866C04E946E}
+	EndGlobalSection
+EndGlobal

MainWindow.xaml

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+<Window x:Class="ColorPicker.MainWindow"
+        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+        xmlns:vm="clr-namespace:ColorPicker.ViewModels"
+        mc:Ignorable="d"
+        Title="MainWindow" Height="600" Width="400">
+    <Window.Resources>
+        <vm:ColorPickerViewModel x:Key="viewModel"/>
+    </Window.Resources>
+    <Grid DataContext="{StaticResource viewModel}">
+        <Grid.RowDefinitions>
+            <RowDefinition Height="4*"/>
+            <RowDefinition Height="*"/>
+            <RowDefinition Height="*"/>
+            <RowDefinition Height="*"/>
+            <RowDefinition Height="*"/>
+        </Grid.RowDefinitions>
+        <Grid.ColumnDefinitions>
+            <ColumnDefinition Width="*" />
+            <ColumnDefinition Width="8*" />
+
+        </Grid.ColumnDefinitions>
+        <Rectangle Fill="{Binding Color}" Margin="10" Name="RGBColor" Grid.Row="0" Grid.ColumnSpan="2"/>
+        <Label Content="Red" Grid.Row="1"/>
+        <Label Content="Green" Grid.Row="2"/>
+        <Label Content="Blue" Grid.Row="3"/>
+        <Slider Name="RedSlider" Maximum="255" Grid.Row="1" Grid.Column="1" Value="{Binding Red}" />
+        <Slider Name="GreenSlider" Maximum="255" Grid.Row="2" Grid.Column="1" Value="{Binding Green}"/>
+        <Slider Name="BlueSlider" Maximum="255" Grid.Row="3" Grid.Column="1" Value="{Binding Blue}"/>
+        <TextBox
+            Name="RGBInHex"
+            Grid.Row="4"
+            Grid.ColumnSpan="2"
+            Margin="10"
+            Text="{Binding Path=HexInput, Mode=TwoWay,  UpdateSourceTrigger=PropertyChanged}"/>
+    </Grid>
+</Window>

MainWindow.xaml.cs

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+using System.Windows;
+
+namespace ColorPicker;
+
+public partial class MainWindow : Window {
+    public MainWindow() {
+        InitializeComponent();
+    }
+}

ViewModels/ColorPicker.cs

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+using CommunityToolkit.Mvvm.ComponentModel;
+using System.Text.RegularExpressions;
+
+namespace ColorPicker.ViewModels;
+
+public partial class ColorPickerViewModel : ObservableObject {
+
+    [ObservableProperty]
+    private byte red = 0, green = 0, blue = 0;
+
+    [ObservableProperty]
+    private string color = "#000000", hexInput = "#000000";
+
+    public ColorPickerViewModel() {
+        calculateColorFromRgb();
+    }
+
+    partial void OnRedChanged(byte value) => calculateColorFromRgb();
+    partial void OnGreenChanged(byte value) => calculateColorFromRgb();
+    partial void OnBlueChanged(byte value) => calculateColorFromRgb();
+
+    partial void OnHexInputChanged(string value) {
+        if (!Regex.IsMatch(value, @"^#[\dA-F]{6}$", RegexOptions.IgnoreCase))
+            return;
+        this.Color = HexInput;
+        calculateRgbFromColor();
+    }
+
+    private void calculateRgbFromColor() {
+        var parse = (int start, int length) => byte.Parse(this.Color.Substring(start, length), System.Globalization.NumberStyles.HexNumber);
+        (this.Red, this.Green, this.Blue) = (parse(1, 2), parse(3, 2), parse(5, 2));
+    }
+
+    private void calculateColorFromRgb() {
+        this.Color = $"#{this.Red:X2}{this.Green:X2}{this.Blue:X2}";
+        this.HexInput = this.Color;
+    }
+
+}

global.json

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+{
+  "sdk": {
+    "version": "7.0.0",
+    "rollForward": "latestMajor",
+    "allowPrerelease": true
+  }
+}