Just name the option 'pretty' for now.

src/compiler/commandLineParser.ts

@@ -21,17 +21,6 @@ namespace ts {
             name: "diagnostics",
             type: "boolean",
         },
-        {
-            name: "diagnosticStyle",
-            paramType: Diagnostics.KIND,
-            description: Diagnostics.Specify_diagnostic_printing_style_Colon_simple_default_or_pretty,
-            type: {
-                "simple": DiagnosticStyle.Simple,
-                "pretty": DiagnosticStyle.Pretty,
-            },
-            error: Diagnostics.Argument_for_diagnosticStyle_must_be_simple_or_pretty,
-            experimental: true,
-        },
         {
             name: "emitBOM",
             type: "boolean"
@@ -162,6 +151,12 @@ namespace ts {
             type: "boolean",
             description: Diagnostics.Do_not_erase_const_enum_declarations_in_generated_code
         },
+        {
+            name: "pretty",
+            paramType: Diagnostics.KIND,
+            description: Diagnostics.Stylize_errors_and_messages_using_colors_if_available_experimental,
+            type: "boolean"
+        },
         {
             name: "project",
             shortName: "p",

src/compiler/diagnosticMessages.json

@@ -2286,14 +2286,10 @@
         "category": "Message",
         "code": 6072
     },
-    "Specify diagnostic printing style: 'simple' (default) or 'pretty'.": {
+    "Stylize errors and messages using colors if available. (experimental)": {
         "category": "Message",
         "code": 6073
     },
-    "Argument for '--diagnosticStyle' must be 'simple' or 'pretty'.": {
-        "category": "Error",
-        "code": 6074
-    },
 
     "Specify JSX code generation: 'preserve' or 'react'": {
         "category": "Message",

src/compiler/tsc.ts

@@ -374,7 +374,7 @@ namespace ts {
                 compilerHost.fileExists = cachedFileExists;
             }
 
-            if (compilerOptions.diagnosticStyle === DiagnosticStyle.Pretty) {
+            if (compilerOptions.pretty) {
                 reportDiagnostic = reportDiagnosticWithColorAndContext;
             }
 

src/compiler/types.ts

@@ -2053,7 +2053,6 @@ namespace ts {
         charset?: string;
         declaration?: boolean;
         diagnostics?: boolean;
-        /* @internal */diagnosticStyle?: DiagnosticStyle;
         emitBOM?: boolean;
         help?: boolean;
         init?: boolean;
@@ -2076,6 +2075,7 @@ namespace ts {
         outFile?: string;
         outDir?: string;
         preserveConstEnums?: boolean;
+        /* @internal */ pretty?: DiagnosticStyle;
         project?: string;
         removeComments?: boolean;
         rootDir?: string;