ColonTest.cs

using SixLabors.Fonts;
using SixLabors.ImageSharp;
using SixLabors.ImageSharp.Processing;
using SixLabors.ImageSharp.Drawing.Processing;
using System;
using SixLabors.ImageSharp.PixelFormats;

namespace ColonTest
{
    class Program
    {
        static void Main(string[] args)
        {
            using Image<Rgba32> image = new Image<Rgba32>(500, 500);
            image.Mutate(c => AddDateStamp(c, "a : b : c", Color.White, @"C:\Windows\Fonts\arial.ttf"));
            image.SaveAsPng(@".\Test.png");
        }

        private static IImageProcessingContext AddDateStamp(IImageProcessingContext processingContext, string dateTime, Color color, string fontPath)
        {
            var imgSize = processingContext.GetCurrentSize();

            float defaultFontSize = 18;
            float defaultResolution = 645;
            float defaultPadding = 10;

            float fontSize = imgSize.Width * defaultFontSize / defaultResolution;
            float padding = imgSize.Width * defaultPadding / defaultResolution;

            FontCollection collection = new FontCollection();
            FontFamily family = collection.Install(fontPath);
            Font font = family.CreateFont(fontSize, FontStyle.Regular);

            var fontRectangle = TextMeasurer.Measure(dateTime, new RendererOptions(font));

            var location = new PointF(imgSize.Width - fontRectangle.Width - padding, imgSize.Height - fontRectangle.Height - padding);
            return processingContext.DrawText(dateTime, font, color, location);
        }
    }
}