A folium element transform the tiles into grayscale (based on https://github.com/Zverik/leaflet-grayscale/)

folium GrayScaleTiles.ipynb

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       "<div style=\"width:100%;\"><div style=\"position:relative;width:100%;height:0;padding-bottom:60%;\"><span style=\"color:#565656\">Make this Notebook Trusted to load map: File -> Trust Notebook</span><iframe src=\"about:blank\" style=\"position:absolute;width:100%;height:100%;left:0;top:0;border:none !important;\" 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      ],
      "text/plain": [
       "<folium.folium.Map at 0x23ac9ee3ac8>"
      ]
     },
     "execution_count": 1,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "import folium\n",
    "from jinja2 import Template\n",
    "\n",
    "class GrayScaleTiles(folium.MacroElement):\n",
    "    \"\"\"Add this element to your map in order to have grayscale tiles.\n",
    "    \n",
    "    Based on https://github.com/Zverik/leaflet-grayscale/\n",
    "    \"\"\"\n",
    "    _name = \"GrayScaleTiles\"\n",
    "    _template = Template(\"\"\"\n",
    "    {% macro header(this, kwargs) %}\n",
    "      <script>\n",
    "        /*\n",
    "         * L.TileLayer.Grayscale is a regular tilelayer with grayscale makeover.\n",
    "         */\n",
    "\n",
    "        L.TileLayer.Grayscale = L.TileLayer.extend({\n",
    "            options: {\n",
    "                quotaRed: 21,\n",
    "                quotaGreen: 71,\n",
    "                quotaBlue: 8,\n",
    "                quotaDividerTune: 0,\n",
    "                quotaDivider: function() {\n",
    "                    return this.quotaRed + this.quotaGreen + this.quotaBlue + this.quotaDividerTune;\n",
    "                }\n",
    "            },\n",
    "\n",
    "            initialize: function (url, options) {\n",
    "                options = options || {}\n",
    "                options.crossOrigin = true;\n",
    "                L.TileLayer.prototype.initialize.call(this, url, options);\n",
    "\n",
    "                this.on('tileload', function(e) {\n",
    "                    this._makeGrayscale(e.tile);\n",
    "                });\n",
    "            },\n",
    "\n",
    "            _createTile: function () {\n",
    "                var tile = L.TileLayer.prototype._createTile.call(this);\n",
    "                tile.crossOrigin = \"Anonymous\";\n",
    "                return tile;\n",
    "            },\n",
    "\n",
    "            _makeGrayscale: function (img) {\n",
    "                if (img.getAttribute('data-grayscaled'))\n",
    "                    return;\n",
    "\n",
    "                img.crossOrigin = '';\n",
    "                var canvas = document.createElement(\"canvas\");\n",
    "                canvas.width = img.width;\n",
    "                canvas.height = img.height;\n",
    "                var ctx = canvas.getContext(\"2d\");\n",
    "                ctx.drawImage(img, 0, 0);\n",
    "\n",
    "                var imgd = ctx.getImageData(0, 0, canvas.width, canvas.height);\n",
    "                var pix = imgd.data;\n",
    "                for (var i = 0, n = pix.length; i < n; i += 4) {\n",
    "                                pix[i] = pix[i + 1] = pix[i + 2] = (this.options.quotaRed * pix[i] + this.options.quotaGreen * pix[i + 1] + this.options.quotaBlue * pix[i + 2]) / this.options.quotaDivider();\n",
    "                }\n",
    "                ctx.putImageData(imgd, 0, 0);\n",
    "                img.setAttribute('data-grayscaled', true);\n",
    "                img.src = canvas.toDataURL();\n",
    "            }\n",
    "        });\n",
    "\n",
    "        L.tileLayer = function (url, options) {\n",
    "            return new L.TileLayer.Grayscale(url, options);\n",
    "        };\n",
    "        </script>\n",
    "        {% endmacro %}\n",
    "    \"\"\")\n",
    "\n",
    "m = folium.Map([45,0], tiles='OpenStreetMap', zoom_start=4).add_child(GrayScaleTiles())\n",
    "m"
   ]
  },
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   "cell_type": "code",
   "execution_count": null,
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   "outputs": [],
   "source": []
  }
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folium GrayScaleTiles.py

import folium
from jinja2 import Template

class GrayScaleTiles(folium.MacroElement):
    """Add this element to your map in order to have grayscale tiles.
    
    Based on https://github.com/Zverik/leaflet-grayscale/
    """
    _name = "GrayScaleTiles"
    _template = Template("""
    {% macro header(this, kwargs) %}
      <script>
        /*
         * L.TileLayer.Grayscale is a regular tilelayer with grayscale makeover.
         */

        L.TileLayer.Grayscale = L.TileLayer.extend({
            options: {
                quotaRed: 21,
                quotaGreen: 71,
                quotaBlue: 8,
                quotaDividerTune: 0,
                quotaDivider: function() {
                    return this.quotaRed + this.quotaGreen + this.quotaBlue + this.quotaDividerTune;
                }
            },

            initialize: function (url, options) {
                options = options || {}
                options.crossOrigin = true;
                L.TileLayer.prototype.initialize.call(this, url, options);

                this.on('tileload', function(e) {
                    this._makeGrayscale(e.tile);
                });
            },

            _createTile: function () {
                var tile = L.TileLayer.prototype._createTile.call(this);
                tile.crossOrigin = "Anonymous";
                return tile;
            },

            _makeGrayscale: function (img) {
                if (img.getAttribute('data-grayscaled'))
                    return;

                img.crossOrigin = '';
                var canvas = document.createElement("canvas");
                canvas.width = img.width;
                canvas.height = img.height;
                var ctx = canvas.getContext("2d");
                ctx.drawImage(img, 0, 0);

                var imgd = ctx.getImageData(0, 0, canvas.width, canvas.height);
                var pix = imgd.data;
                for (var i = 0, n = pix.length; i < n; i += 4) {
                                pix[i] = pix[i + 1] = pix[i + 2] = (this.options.quotaRed * pix[i] + this.options.quotaGreen * pix[i + 1] + this.options.quotaBlue * pix[i + 2]) / this.options.quotaDivider();
                }
                ctx.putImageData(imgd, 0, 0);
                img.setAttribute('data-grayscaled', true);
                img.src = canvas.toDataURL();
            }
        });

        L.tileLayer = function (url, options) {
            return new L.TileLayer.Grayscale(url, options);
        };
        </script>
        {% endmacro %}
    """)

m = folium.Map([45,0], tiles='OpenStreetMap', zoom_start=4).add_child(GrayScaleTiles())
m