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I am creating test images for an electronic sign.

The image was created in GIMP 2.10.8 and was declared an RGB image
before anything was drawn.

It consists of 24 pairs of rows, one pixel in height and 112 pixels long. Each pair consists of a black row and a color row.

How to make a single test pattern and then change the color of the color row with a simple step(s) to make multiple test images.

I created the original image using a white background upon which I drew black foreground lines with a pencil..

When sent to the LED sign, the test image should turn on, in the vertical dimension only, every other pixel so that Red and Green and Blue LEDs produce white if the pixel is on. Otherwise the pixel is off.

Now i would like to create another test pattern replacing white with some other color.

I could select each white row and change its color using a bucket fill, but there must be a better way that reduces making all those selections. Since I am using only the foreground color and background color I thought maybe I could change the background color and all the spatial regions that were the original background color would automatically change. However that did not work.

In looking around it seems some people use "color to alpha plugin".
The only problem is I can not find such a plugin....not even a grayed out plugin.

So FIRST....Am i pursuing the most appropriate method of changing the white background (that is how the white originally was formed) to another color by using the color to alpha plugin. IF not please offer an alternative approach.

if so, please help me gain access to the color to alpha plugin. As mentioned earlier, I declared the image mode as RGB before I drew a single line.

One last request....It would be nice if I could reverse the black lines and make them white lines and repeat the above changes. That would enable me to test the pixels are not tested in the above test pattern. I also would like to change the horizontal lines to vertical lines. Perhaps there is an easy way to do that.

Keep in mind, I am trying to learn GIMP so I am willing to invest the time to learn the technique which I can use in the future for more complex images.

Thank you for any input you can offer me.

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