diff --git a/README.md b/README.md
index 07947c8..add5ad0 100644
--- a/README.md
+++ b/README.md
@@ -22,7 +22,7 @@ make clean build install
 ## CLI arguments
 
 ```txt
-usage: margen [--help] [-v] [-q] [-w=WIDTH] [-h=HEIGHT] [-o=PATH] [--seed=SEED][-p=PIXEL_SIZE] [-s=SLOPE] [-c=R,G,B] [-va=R,G,B] [-vr=VAR_RANGE] [-r=ROTATION]
+usage: margen [--help] [-v] [-q] [-w=WIDTH] [-h=HEIGHT] [-o=PATH] [--seed=SEED][-p=PIXEL_SIZE] [-s=SLOPE] [-c=R,G,B] [-va=R,G,B] [-vr=VAR_RANGE] [-r=ROTATION] [-m]
 
 generate a marble-like pattern bitmap image, blazing fast.
 
@@ -39,8 +39,9 @@ options:
   -va, --variation   fixed variation [0-255,0-255,0-255] (default: random)
   -vr, --var-range   random variation range [0-255] (default: 30)
   -r, --rotation     start corner rotation [0-3] (default: random)
+  -m, --monochrome   grayscale generation
 ```
 
 ## TODO
 
-- fix monochrome
\ No newline at end of file
+- configure, make, make install
\ No newline at end of file
diff --git a/src/args.c b/src/args.c
index dac1785..f1c939d 100644
--- a/src/args.c
+++ b/src/args.c
@@ -40,8 +40,7 @@ void print_help(int status_code) {
        "random)\n"
        "  -vr, --var-range   random variation range [0-255] (default: 30)\n"
        "  -r, --rotation    start corner rotation [0-3] (default: random)\n"
-       //  "  -m, --monochrome   black & white generation\n"
-  );
+       "  -m, --monochrome   grayscale generation\n");
   exit(status_code);
 }
 
diff --git a/src/bmp.c b/src/bmp.c
index 974bd1d..bf70359 100644
--- a/src/bmp.c
+++ b/src/bmp.c
@@ -32,23 +32,36 @@ unsigned char *bmp_header(unsigned short width, unsigned short height,
   unsigned char *output = (unsigned char *)malloc(HEADER_SIZE);
   unsigned int data_length = ((unsigned int)width) * ((unsigned int)height) *
                              ((unsigned int)color_depth);
-  write_str(output, 0x00, 0x02, (unsigned char *)"BM"); // 0x00(2) BM
-  write_num(output, 0x02, 0x04,
-            HEADER_SIZE + data_length);       // 0x02(4) file size
-  write_nul(output, 0x06, 0x04);              // 0x06(4) application reserved
-  write_num(output, 0x0A, 0x04, HEADER_SIZE); // 0x0A(4) data offset
-  write_num(output, 0x0E, 0x04, 40);          // 0x0E(4) DIB header size
-  write_num(output, 0x12, 0x04, width);       // 0x12(4) width
-  write_num(output, 0x16, 0x04, height);      // 0x16(4) height
-  write_num(output, 0x1A, 0x04, 1);           // 0x1A(2) color panes
-  write_num(output, 0x1C, 0x02, color_depth * 8); // 0x1C(2) bits per pixel
-  write_nul(output, 0x1E, 0x04); // 0x1E(4) BI_RGB, no compression
-  write_num(output, 0x22, 0x04,
-            data_length);              // 0x22(4) size of raw bitmap data
-  write_num(output, 0x26, 0x04, 2835); // 0x26(4) horizontal print resolution
-  write_num(output, 0x2A, 0x04, 2835); // 0x2A(4) vertical print resolution
-  write_nul(output, 0x2E, 0x04);       // 0x2E(4) color in palette
-  write_nul(output, 0x32, 0x04);       // 0x32(4) 0 important colors
+  // 0x00(2) BM
+  write_str(output, 0x00, 0x02, (unsigned char *)"BM");
+  // 0x02(4) file size
+  write_num(output, 0x02, 0x04, HEADER_SIZE + data_length);
+  // 0x06(4) application reserved
+  write_nul(output, 0x06, 0x04);
+  // 0x0A(4) data offset
+  write_num(output, 0x0A, 0x04, HEADER_SIZE);
+  // 0x0E(4) DIB header size
+  write_num(output, 0x0E, 0x04, 40);
+  // 0x12(4) width
+  write_num(output, 0x12, 0x04, width);
+  // 0x16(4) height
+  write_num(output, 0x16, 0x04, height);
+  // 0x1A(2) color panes
+  write_num(output, 0x1A, 0x04, 1);
+  // 0x1C(2) bits per pixel
+  write_num(output, 0x1C, 0x02, color_depth * 8);
+  // 0x1E(4) BI_RGB, no compression
+  write_nul(output, 0x1E, 0x04);
+  // 0x22(4) size of raw bitmap data
+  write_num(output, 0x22, 0x04, data_length);
+  // 0x26(4) horizontal print resolution
+  write_num(output, 0x26, 0x04, 2835);
+  // 0x2A(4) vertical print resolution
+  write_num(output, 0x2A, 0x04, 2835);
+  // 0x2E(4) color in palette (0)
+  write_nul(output, 0x2E, 0x04);
+  // 0x32(4) important colors (0)
+  write_nul(output, 0x32, 0x04);
 
   return output;
 }
diff --git a/src/generator.c b/src/generator.c
index a7b5a03..a712bd8 100644
--- a/src/generator.c
+++ b/src/generator.c
@@ -7,6 +7,8 @@
 #include <stdlib.h>
 #include <time.h>
 
+#define BMP_COLOR_DEPTH 3
+
 parameters global_params;
 float slope;
 unsigned char color_depth;
@@ -47,7 +49,7 @@ void generate_bmp_line(unsigned short y, unsigned char *data_buffer,
     generate_line();
   }
   for (i = 0; i < len; i++) {
-    x = i / (color_depth * global_params.size);
+    x = i / (BMP_COLOR_DEPTH * global_params.size);
     data_buffer[(global_params.rotation / 2) == 1 ? i : (len - i - 1)] =
         current_line[x * color_depth + (i % color_depth)];
   }
@@ -106,7 +108,7 @@ void generate(parameters params) {
   }
   clock_t start = clock();
   init(params);
-  bmp_generate(params.width, params.height, color_depth,
+  bmp_generate(params.width, params.height, BMP_COLOR_DEPTH,
                params.rotation % 2 == 1, params.file_path, generate_bmp_line);
   clean();
   print_time(params, start);