Gets the most dominant color from a image and saves it to a property with the name of "umbacoDominantColor"

DominantColorEventHandler.cs

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Linq;
using System.Web;

using ImageProcessor;
using ImageProcessor.Imaging;

using Newtonsoft.Json;
using Umbraco.Core;
using Umbraco.Core.Configuration;
using Umbraco.Core.Configuration.UmbracoSettings;
using Umbraco.Core.Models;
using Umbraco.Core.Services;
using Umbraco.Core.Logging;

namespace Umbraco.Web
{
    public class ApplicationEvents: IApplicationEventHandler
    {
        #region Events
        public void OnApplicationInitialized(UmbracoApplicationBase umbracoApplication, ApplicationContext applicationContext)
        {
            throw new NotImplementedException();
        }

        public void OnApplicationStarted(UmbracoApplicationBase umbracoApplication, ApplicationContext applicationContext)
        {
        }

        public void OnApplicationStarting(UmbracoApplicationBase umbracoApplication, ApplicationContext applicationContext)
        {
            // Tap into the Saving event
            MediaService.Saving += MediaService_Saving;
        }
        #endregion

        void MediaService_Saving (IMediaService sender, Core.Events.SaveEventArgs<IMedia> e)
        {
            IContentSection contentSection = UmbracoConfig.For.UmbracoSettings().Content;
            IEnumerable<string> supportedTypes = contentSection.ImageFileTypes.ToList();

            foreach( IMedia media in e.SavedEntities )
            {
                if( media.HasProperty( "umbracoFile" ) && media.HasProperty( "umbracoDominantColor" ) )
                {
                    string extention = media.GetValue<string>( "umbracoExtension" );
                    if( supportedTypes.InvariantContains( extention ) )
                    {
                        Color dominantColor = GetDominantColorFromMedia( media );
                        string hex = ConvertColorToHex( dominantColor );
                        media.SetValue( "umbracoDominantColor", hex );
                    }
                }
            }
        }

        private Color GetDominantColorFromMedia (IMedia media)
        {
            string path = GetImagePathFromMedia( media );
            Color dominantColor = GetDominantColorFromImagePath( path );
            return dominantColor;
        }

        private Color GetDominantColorFromImagePath (string path)
        {
            try
            {
                // Resize the image to 1px wide
                using( ImageFactory imageFactory = new ImageFactory( false ) )
                {
                    var size = new System.Drawing.Size() { Width = 1 };
                    var resizeLayer = new ResizeLayer( size : size, resizeMode : ResizeMode.Max );
                    var image = imageFactory.Load( path ).Resize( resizeLayer ).Image;

                    // Now get the only color left, dominant color.
                    using( Bitmap bmp = new Bitmap( image ) )
                    {
                        return bmp.GetPixel( 0, 0 );
                    }
                } // END of ImageFactory
            }
            catch( Exception )
            {
                throw new NotImplementedException( "Unable to get dominant color from image" ); ;
            }
        }

        private string ConvertColorToHex (System.Drawing.Color c)
        {
            return "#" + c.R.ToString( "X2" ) + c.G.ToString( "X2" ) + c.B.ToString( "X2" );
        }

        private string GetImagePathFromMedia (IMedia media)
        {
            string value = media.GetValue<string>("umbracoFile");
            string src = GetSrcFromMedia(value);

            if (!string.IsNullOrEmpty(src))
            {
                return HttpContext.Current.Server.MapPath(src);
            }

            return string.Empty;
        }

        private string GetSrcFromMedia(string value)
        {
            string src;
            try
            {
                var imageCropDateSet = JsonConvert.DeserializeObject<Umbraco.Web.Models.ImageCropDataSet>(value);
                src = imageCropDateSet.Src;
            }
            catch (Exception ex)
            {
                LogHelper.Error(this.GetType(), "Failed to parse umbracoFile as ImageCropper", ex);
                src = value;
            }

            return src;
        }

        
    }
}

DominantColorExtractorComposer.cs

using ImageProcessor;
using ImageProcessor.Imaging;
using Newtonsoft.Json;
using System;
using System.Collections.Generic;
using System.Drawing;
using System.Linq;
using System.Web;
using Umbraco.Core;
using Umbraco.Core.Composing;
using Umbraco.Core.Configuration.UmbracoSettings;
using Umbraco.Core.Models;
using Umbraco.Core.PropertyEditors.ValueConverters;
using Umbraco.Core.Services;
using Umbraco.Core.Services.Implement;

namespace Umbraco.Web
{
    public class DominantColorExtractorComposer : IUserComposer
    {
        #region Events
        public void Compose(Composition composition)
        {
            MediaService.Saving += MediaService_Saving;
        }
        #endregion

        void MediaService_Saving(IMediaService sender, Core.Events.SaveEventArgs<IMedia> e)
        {
            IContentSection contentSection = Current.Configs.Settings().Content;
            IEnumerable<string> supportedTypes = contentSection.ImageFileTypes.ToList();

            foreach (IMedia media in e.SavedEntities)
            {
                if (media.HasProperty("umbracoFile") && media.HasProperty("umbracoDominantColor"))
                {
                    string extention = media.GetValue<string>("umbracoExtension");
                    if (supportedTypes.InvariantContains(extention))
                    {
                        Color dominantColor = GetDominantColorFromMedia(media);
                        string hex = ConvertColorToHex(dominantColor);
                        media.SetValue("umbracoDominantColor", hex);
                    }
                }
            }
        }

        private Color GetDominantColorFromMedia(IMedia media)
        {
            string path = GetImagePathFromMedia(media);
            Color dominantColor = GetDominantColorFromImagePath(path);
            return dominantColor;
        }

        private Color GetDominantColorFromImagePath(string path)
        {
            try
            {
                // Resize the image to 1px wide
                using (ImageFactory imageFactory = new ImageFactory(false))
                {
                    var size = new System.Drawing.Size() { Width = 3, height = 3 };
                    var resizeLayer = new ResizeLayer(size: size, resizeMode: ResizeMode.Max);
                    var image = imageFactory.Load(path).Resize(resizeLayer).Image;

                    // Now get the only color left, dominant color.
                    using (Bitmap bmp = new Bitmap(image))
                    {
                        return bmp.GetPixel(1, 1);
                    }
                } // END of ImageFactory
            }
            catch (Exception)
            {
                throw new NotImplementedException("Unable to get dominant color from image"); ;
            }
        }

        private string ConvertColorToHex(System.Drawing.Color c)
        {
            return "#" + c.R.ToString("X2") + c.G.ToString("X2") + c.B.ToString("X2");
        }

        private string GetImagePathFromMedia(IMedia media)
        {
            string value = media.GetValue<string>("umbracoFile");
            string src = GetSrcFromMedia(value);

            if (!string.IsNullOrEmpty(src))
            {
                return HttpContext.Current.Server.MapPath(src);
            }

            return string.Empty;
        }

        private string GetSrcFromMedia(string value)
        {
            string src;
            try
            {
                var imageCropDateSet = JsonConvert.DeserializeObject<ImageCropperValue>(value);
                src = imageCropDateSet.Src;
            }
            catch (Exception ex)
            {
                Current.Logger.Error(this.GetType(), "Failed to parse umbracoFile as ImageCropper", ex);
                src = value;
            }

            return src;
        }

        
    }
}