This question is a little tricky since it talks about both purple light and a mixture of blue and red light. There is such a thing as purple light. It has a wavelength around 400 nanometers. Blue light has a wavelength around 450 nanometers and red light has a wavelength of 700 nanometers. Curiously, the human eye can be fooled into interpreting combinations of red and blue as purple.
If the light were really purple, it would be diffracted at an angle related to the wavelength of purple light and the spacing of the diffraction grating. All of the purple light would be diffracted at the same angle.
If the light is actually made up of both red and blue, these colors will diffract at different angles. The diffraction pattern will split into red and blue light. If the light is coming from a single point or line, it will split into two points or lines when viewed through a diffraction grating.