You can use the canvas plugin to reference pixels.
If you're sampling only a few pixels you can use the Canvas.rgbaAt(x,y) expression to get color values like this "rgba(0,0,0,0)", "rgba(255,255,0,1)".
I also added another expression Canvas.AsJSON which is the entire image in a format that can be loaded into an array with a load action. It is a very quick WIP, so it's not that intuitive. The array size ends up being (1,1,Canvas.Width*Canvas.Height*4).
To access a pixel use this expression:
Array.At(0, 0, (y*Canvas.Width + x)*4 + component)
Where x,y is the pixel position and component is 0 for red, 1 for green, 2 for blue and 3 for alpha. Each component will be a number in the range of 0 to 255.