文字格式设置:

字体颜色设置:

测试:

Program.cs:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Windows.Forms;

namespace ColorDialog
{
    static class Program
    {
        /// <summary>
        /// 应用程序的主入口点。
        /// </summary>
        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            Application.Run(new Form1());
        }
    }
}

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace ColorDialog
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            DialogResult dr = colorDialog1.ShowDialog(); //显示选择文字颜色对话框
            if(dr==DialogResult.OK)
            {
                richTextBox1.SelectionColor = colorDialog1.Color;
                //设置 richTextbox1 中的文字颜色

            }
        }

        private void button2_Click(object sender, EventArgs e)
        {
            DialogResult dr = fontDialog1.ShowDialog();
            if(dr==DialogResult.OK)
            {
                richTextBox1.SelectionFont = fontDialog1.Font;
            }
        }
    }
}

namespace ColorDialog
{
    partial class Form1
    {
        /// <summary>
        /// 必需的设计器变量。
        /// </summary>
        private System.ComponentModel.IContainer components = null;

        /// <summary>
        /// 清理所有正在使用的资源。
        /// </summary>
        /// <param name="disposing">如果应释放托管资源，为 true；否则为 false。</param>
        protected override void Dispose(bool disposing)
        {
            if (disposing && (components != null))
            {
                components.Dispose();
            }
            base.Dispose(disposing);
        }

        #region Windows 窗体设计器生成的代码

        /// <summary>
        /// 设计器支持所需的方法 - 不要
        /// 使用代码编辑器修改此方法的内容。
        /// </summary>
        private void InitializeComponent()
        {
            this.colorDialog1 = new System.Windows.Forms.ColorDialog();
            this.richTextBox1 = new System.Windows.Forms.RichTextBox();
            this.button1 = new System.Windows.Forms.Button();
            this.button2 = new System.Windows.Forms.Button();
            this.fontDialog1 = new System.Windows.Forms.FontDialog();
            this.SuspendLayout();
            // 
            // richTextBox1
            // 
            this.richTextBox1.Location = new System.Drawing.Point(1, 69);
            this.richTextBox1.Name = "richTextBox1";
            this.richTextBox1.Size = new System.Drawing.Size(292, 153);
            this.richTextBox1.TabIndex = 0;
            this.richTextBox1.Text = "       哈哈哈";
            // 
            // button1
            // 
            this.button1.Location = new System.Drawing.Point(162, 25);
            this.button1.Name = "button1";
            this.button1.Size = new System.Drawing.Size(75, 23);
            this.button1.TabIndex = 1;
            this.button1.Text = "选择颜色";
            this.button1.UseVisualStyleBackColor = true;
            this.button1.Click += new System.EventHandler(this.button1_Click);
            // 
            // button2
            // 
            this.button2.Location = new System.Drawing.Point(55, 25);
            this.button2.Name = "button2";
            this.button2.Size = new System.Drawing.Size(75, 23);
            this.button2.TabIndex = 2;
            this.button2.Text = "选择字体";
            this.button2.UseVisualStyleBackColor = true;
            this.button2.Click += new System.EventHandler(this.button2_Click);
            // 
            // Form1
            // 
            this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 12F);
            this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
            this.ClientSize = new System.Drawing.Size(290, 218);
            this.Controls.Add(this.button2);
            this.Controls.Add(this.button1);
            this.Controls.Add(this.richTextBox1);
            this.Name = "Form1";
            this.Text = "Form1";
            this.ResumeLayout(false);

        }

        #endregion

        private System.Windows.Forms.ColorDialog colorDialog1;
        private System.Windows.Forms.RichTextBox richTextBox1;
        private System.Windows.Forms.Button button1;
        private System.Windows.Forms.Button button2;
        private System.Windows.Forms.FontDialog fontDialog1;
    }
}