Modify Fill Colour from Outside a Paint Event Handler in C# and Visual Studio 2012

Question

What I Would Like to Do
Create and fill a rectangle, initially with a grey colour, then randomize that rectangle's fill colour with the press of a button.
What I Cannot Seem to Do
Modify properties of the aforementioned rectangle after it is initially drawn.

Random Colour Generator

public static readonly Random random = new Random();
public static readonly object syncLock = new object();
public static int RandomNumber( int min, int max )
{
  lock(syncLock) {
    return random.Next(min, max);
  }
}
public Color randColourPicker() {
  Color randColour = Color.FromArgb(RandomNumber(0, 255), RandomNumber(0, 255), RandomNumber(0, 255));
  return randColour;
}

Some randColourPicker() Calls

public void randomizeColours() {
    this.BackColor = randColourPicker();
    clrRandLabel.ForeColor = randColourPicker();
    randomizeButton.BackColor = randColourPicker();
    randomizeButton.ForeColor = randColourPicker();
    loopCheckbox.ForeColor = randColourPicker();
}

Rectangle Painting

private void clrRandForm_Paint( object sender, PaintEventArgs e ) {
    int x = 266;
    int y = 105;
    int width = 274;
    int height = 172;

    Rectangle clrRect = new Rectangle(x, y, width, height);
    SolidBrush greyBrush = new SolidBrush(Color.FromArgb(75, 75, 75));
    SolidBrush randBrush = new SolidBrush(randColourPicker());
    Graphics clrGraphic = mainBox.CreateGraphics();

    clrGraphic.FillEllipse(greyBrush, clrRect);

    greyBrush.Dispose();
    randBrush.Dispose();
    clrGraphic.Dispose();
}

More Info
I have a button, and it's click event is bound to execute the randomizeColours() call, which in turn, swaps around the colours of various elements of the UI. I also have a check box that, when checked, runs through randomizeColours() on loop with a timer. I would like to do something similar, but with the fill colour of clrRect, but I cannot seem to figure out how I would access clrGraphic.FillRectangle(greyBrush, clrRect) outside of the PaintEvent function, so I can modify it to clrGraphic.FillRectangle(randBrush, clrRect). randBrush is a brush I created with a random colour using randColourPicker().

Programs Used
Microsoft Visual Studio Express 2012

P.S.
Sorry if this question is a duplicate, I couldn't quite figure out what to search for... Also, sorry if my code is cringe-worthy to some, I'm not really that good with the OOP approach or C-styled languages. :)

Answer 1

To change the Rectangle color you have to draw it again. You can keep your paint event function but with some changes. First, declare a global boolean First_time (initially set with true) that will check if you you want the greybrush or the Randbrush.

boolean First_Time;

Then in your Paint event :

private void clrRandForm_Paint( object sender, PaintEventArgs e ) {
    int x = 266;
    int y = 105;
    int width = 274;
    int height = 172;

    Rectangle clrRect = new Rectangle(x, y, width, height);
    SolidBrush greyBrush = new SolidBrush(Color.FromArgb(75, 75, 75));
    SolidBrush randBrush = new SolidBrush(randColourPicker());
    Graphics clrGraphic = mainBox.CreateGraphics();

    if(First_time)
        clrGraphic.FillEllipse(greyBrush, clrRect);
    else
        clrGraphic.FillEllipse(randBrush, clrRect);
    First_Time = false;

    greyBrush.Dispose();
    randBrush.Dispose();
    clrGraphic.Dispose();
}

And then in your button event, call:

this.Invalidate(); // or this.Refresh()

So that the Paint event is called again xD. I hope my advise was usefull.