public partial class ucZeleny : UserControl
{
    public ucModry Modry { get; set; }

    public ucZeleny()
    {
        InitializeComponent();
    }

    void btnSkryjZobraz_Click(object sender, EventArgs e)
    {
        Modry.Visible = !Modry.Visible;
    }
}

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();

        ucZeleny1.Modry = ucModry1;
    }
}