If no encoding mode is specified in the options, RGB will be used.

src/ImageSharp/Formats/Tiff/Compression/Compressors/TiffLzwEncoder.cs

@@ -260,14 +260,8 @@ namespace SixLabors.ImageSharp.Formats.Experimental.Tiff.Utils
             this.nextValidCode = EoiCode + 1;
         }
 
-        private static int MaxValue(int codeLen)
-        {
-            return (1 << codeLen) - 1;
-        }
+        private static int MaxValue(int codeLen) => (1 << codeLen) - 1;
 
-        private static int BitmaskFor(int bits)
-        {
-            return MaxValue(bits);
-        }
+        private static int BitmaskFor(int bits) => MaxValue(bits);
     }
 }

src/ImageSharp/Formats/Tiff/ITiffEncoderOptions.cs

@@ -24,6 +24,7 @@ namespace SixLabors.ImageSharp.Formats.Experimental.Tiff
 
         /// <summary>
         /// Gets the encoding mode to use. RGB, RGB with color palette or gray.
+        /// If no mode is specified in the options, RGB will be used.
         /// </summary>
         TiffEncodingMode Mode { get; }
 

src/ImageSharp/Formats/Tiff/TiffEncoderCore.cs

@@ -82,6 +82,7 @@ namespace SixLabors.ImageSharp.Formats.Experimental.Tiff
 
         /// <summary>
         /// Gets the encoding mode to use. RGB, RGB with color palette or gray.
+        /// If no mode is specified in the options, RGB will be used.
         /// </summary>
         internal TiffEncodingMode Mode { get; private set; }
 
@@ -118,6 +119,10 @@ namespace SixLabors.ImageSharp.Formats.Experimental.Tiff
                 {
                     this.Mode = TiffEncodingMode.BiColor;
                 }
+                else
+                {
+                    this.Mode = TiffEncodingMode.Rgb;
+                }
             }
 
             this.SetPhotometricInterpretation();