How can I use a Shader in XNA to color single pixels?

I have a standard 800x600 window in my XNA project. My goal is to color each individual pixel based on a rectangle array which holds boolean values. Currently I am using a 1x1 Texture and drawing each sprite in my array.

I am very new to XNA and come from a GDI background, so I am doing what I would have done in GDI, but it doesn't scale very well. I have been told in another question to use a Shader, but after much research, I still haven't been able to find out how to accomplish this goal.

My application loops through the X and Y coordinates of my rectangular array, does calculations based on each value, and reassigns/moves the array around. At the end, I need to update my "Canvas" with the new values. A smaller sample of my array would look like:

0,0,0,0,0,0,0
0,0,0,0,0,0,0
0,0,0,0,0,0,0
1,1,1,1,1,1,1
1,1,1,1,1,1,1

How can I use a shader to color each pixel?

A very simplified version of the calculations would be:

        for (int y = _horizon; y >= 0; y--)  // _horizon is my ending point
        {
            for (int x = _width; x >= 0; x--) // _width is obviously my x length.
            {
                if (grains[x, y] > 0)
                {
                    if (grains[x, y + 1] == 0)
                    {
                        grains[x, y + 1] = grains[x, y];
                        grains[x, y] = 0;
                    }
                }
            }
        }

..each time the update method is called, the calculations are performed and in example of the above loop, an update may look like:

Initial:

0,0,0,1,0,0,0
0,0,0,0,0,0,0
0,0,0,0,0,0,0
1,1,1,0,1,1,1
1,1,1,1,1,1,1

First:

0,0,0,0,0,0,0
0,0,0,1,0,0,0
0,0,0,0,0,0,0
1,1,1,0,1,1,1
1,1,1,1,1,1,1

Second:

0,0,0,0,0,0,0
0,0,0,0,0,0,0
0,0,0,1,0,0,0
1,1,1,0,1,1,1
1,1,1,1,1,1,1

Final: