ThemaColorPickerApiController.cs

namespace MyWebsite.Controllers.PropEditors
{
    [Umbraco.Web.Mvc.PluginController("Custom")]
    public class ThemaColorPickerApiController: UmbracoAuthorizedApiController
    {

        public IEnumerable<Color> GetAvailableColors(int page) {
            var colors = new List<Color>();

            var themePage = Services.ContentService.GetAncestors(page).FirstOrDefault(x => x.HasProperty("theme"));
            
            if (themePage != null)
            {
                var palette = JsonConvert.DeserializeObject<ColorPalette>(themePage.GetValue("theme").ToString());

                foreach (var color in palette.Colors)
                    colors.Add(color);
            }
            return colors; 
            
        }
    }
}