MainForm.cs

using System;
using System.Drawing;
using System.Globalization;
using System.Windows.Forms;

namespace SixthTask
{
    public partial class MainForm : Form
    {
        public MainForm()
        {
            InitializeComponent();
        }

        private void nudColor_ValueChanged(object sender, EventArgs e)
        {
            var numericUpDown = (NumericUpDown) sender;
            var backgroundColor = Color.FromArgb((int) nudRed.Value, (int) nudGreen.Value, (int) nudBlue.Value);
            var foregroundColor = Color.FromArgb(255 - (int) nudRed.Value, 255 - (int) nudGreen.Value,
                                                 255 - (int) nudBlue.Value);

            BackColor = backgroundColor;
            lblName.ForeColor = foregroundColor;

            if ((string) numericUpDown.Tag == "red")
            {
                Text = numericUpDown.Value.ToString(CultureInfo.CurrentCulture);
            }
        }
    }
}

MainForm.Designer.cs

namespace SixthTask
{
    partial class MainForm
    {
        /// <summary>
        /// Required designer variable.
        /// </summary>
        private System.ComponentModel.IContainer components = null;

        /// <summary>
        /// Clean up any resources being used.
        /// </summary>
        /// <param name="disposing">true if managed resources should be disposed; otherwise, false.</param>
        protected override void Dispose(bool disposing)
        {
            if (disposing && (components != null))
            {
                components.Dispose();
            }
            base.Dispose(disposing);
        }

        #region Windows Form Designer generated code

        /// <summary>
        /// Required method for Designer support - do not modify
        /// the contents of this method with the code editor.
        /// </summary>
        private void InitializeComponent()
        {
            this.label1 = new System.Windows.Forms.Label();
            this.nudRed = new System.Windows.Forms.NumericUpDown();
            this.label2 = new System.Windows.Forms.Label();
            this.nudGreen = new System.Windows.Forms.NumericUpDown();
            this.label3 = new System.Windows.Forms.Label();
            this.nudBlue = new System.Windows.Forms.NumericUpDown();
            this.lblName = new System.Windows.Forms.Label();
            ((System.ComponentModel.ISupportInitialize)(this.nudRed)).BeginInit();
            ((System.ComponentModel.ISupportInitialize)(this.nudGreen)).BeginInit();
            ((System.ComponentModel.ISupportInitialize)(this.nudBlue)).BeginInit();
            this.SuspendLayout();
            // 
            // label1
            // 
            this.label1.AutoSize = true;
            this.label1.Location = new System.Drawing.Point(12, 14);
            this.label1.Name = "label1";
            this.label1.Size = new System.Drawing.Size(30, 13);
            this.label1.TabIndex = 0;
            this.label1.Text = "Red:";
            // 
            // nudRed
            // 
            this.nudRed.Location = new System.Drawing.Point(57, 12);
            this.nudRed.Maximum = new decimal(new int[] {
            255,
            0,
            0,
            0});
            this.nudRed.Name = "nudRed";
            this.nudRed.Size = new System.Drawing.Size(120, 20);
            this.nudRed.TabIndex = 1;
            this.nudRed.Tag = "red";
            this.nudRed.ValueChanged += new System.EventHandler(this.nudColor_ValueChanged);
            // 
            // label2
            // 
            this.label2.AutoSize = true;
            this.label2.Location = new System.Drawing.Point(12, 40);
            this.label2.Name = "label2";
            this.label2.Size = new System.Drawing.Size(39, 13);
            this.label2.TabIndex = 2;
            this.label2.Text = "Green:";
            // 
            // nudGreen
            // 
            this.nudGreen.Location = new System.Drawing.Point(57, 38);
            this.nudGreen.Maximum = new decimal(new int[] {
            255,
            0,
            0,
            0});
            this.nudGreen.Name = "nudGreen";
            this.nudGreen.Size = new System.Drawing.Size(120, 20);
            this.nudGreen.TabIndex = 3;
            this.nudGreen.Tag = "green";
            this.nudGreen.ValueChanged += new System.EventHandler(this.nudColor_ValueChanged);
            // 
            // label3
            // 
            this.label3.AutoSize = true;
            this.label3.Location = new System.Drawing.Point(12, 66);
            this.label3.Name = "label3";
            this.label3.Size = new System.Drawing.Size(31, 13);
            this.label3.TabIndex = 4;
            this.label3.Text = "Blue:";
            // 
            // nudBlue
            // 
            this.nudBlue.Location = new System.Drawing.Point(57, 64);
            this.nudBlue.Maximum = new decimal(new int[] {
            255,
            0,
            0,
            0});
            this.nudBlue.Name = "nudBlue";
            this.nudBlue.Size = new System.Drawing.Size(120, 20);
            this.nudBlue.TabIndex = 5;
            this.nudBlue.Tag = "blue";
            this.nudBlue.ValueChanged += new System.EventHandler(this.nudColor_ValueChanged);
            // 
            // lblName
            // 
            this.lblName.Font = new System.Drawing.Font("Microsoft Sans Serif", 27.75F, System.Drawing.FontStyle.Regular, System.Drawing.GraphicsUnit.Point, ((byte)(238)));
            this.lblName.Location = new System.Drawing.Point(15, 87);
            this.lblName.Name = "lblName";
            this.lblName.Size = new System.Drawing.Size(162, 42);
            this.lblName.TabIndex = 6;
            this.lblName.Text = "Milan Vít";
            this.lblName.TextAlign = System.Drawing.ContentAlignment.MiddleCenter;
            // 
            // MainForm
            // 
            this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F);
            this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
            this.ClientSize = new System.Drawing.Size(189, 138);
            this.Controls.Add(this.lblName);
            this.Controls.Add(this.nudBlue);
            this.Controls.Add(this.label3);
            this.Controls.Add(this.nudGreen);
            this.Controls.Add(this.label2);
            this.Controls.Add(this.nudRed);
            this.Controls.Add(this.label1);
            this.MaximizeBox = false;
            this.MinimizeBox = false;
            this.Name = "MainForm";
            this.Text = "Sixth Task";
            ((System.ComponentModel.ISupportInitialize)(this.nudRed)).EndInit();
            ((System.ComponentModel.ISupportInitialize)(this.nudGreen)).EndInit();
            ((System.ComponentModel.ISupportInitialize)(this.nudBlue)).EndInit();
            this.ResumeLayout(false);
            this.PerformLayout();

        }

        #endregion

        private System.Windows.Forms.Label label1;
        private System.Windows.Forms.NumericUpDown nudRed;
        private System.Windows.Forms.Label label2;
        private System.Windows.Forms.NumericUpDown nudGreen;
        private System.Windows.Forms.Label label3;
        private System.Windows.Forms.NumericUpDown nudBlue;
        private System.Windows.Forms.Label lblName;
    }
}

Program.cs

using System;
using System.Windows.Forms;

namespace SixthTask
{
    static class Program
    {
        /// <summary>
        /// The main entry point for the application.
        /// </summary>
        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            Application.Run(new MainForm());
        }
    }
}