Detecting a certain amount of violet in an image
Try using HSV/HSL to represent color instead of RGB.

You can then define violet as some hue range, e.g. 250° to 310° and similarly for saturation and lightness/value.



Always use a using when you have tempoary disposable resources like a Bitmap



Variables in C# are camelCase:

Boolean colourFound = false;
Bitmap selectedImage = new Bitmap(picture.Image);
Color selectedPixel;




Try to limit the scope of variables, move the declaration of selectedPixel to where you currently assign it.