My friend well, this question been asked 6 years ago, I think definitely you solve your problem, so for the people who stuck with the same problem you can fix via below tactics.

Updated ClearCanvas Function:

function ClearCanvas(sColour) {
    DebugMessage("Clear Canvas");
    const context = oCanvas.getContext('2d');

    // Clear the canvas
    context.clearRect(0, 0, oCanvas.width, oCanvas.height);

    // Set the background color
    oCanvas.style.background = sColour; // Update the canvas background
}

A few things to be kept in mind

1. Ensure oCanvas is properly defined and refers to the element.
2. Instead of "____", pass a valid CSS color string (e.g., "red", "#ff0000", "rgba(255, 0, 0, 0.5)", etc.).

I hope it will help others to get rid of these problem