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

folium GrayScaleTiles.ipynb

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