I am looking for a way to make a thin horizontal strip (image) that has 3 different colors on it.
It will be used as a background and repeated vertically to create the illusion of having 3 differently colored columns as part of my page layout.
I am on a MacBook and do not have Photoshop (or any $$$) and am looking for an effective solution.
TomTees
Try this free online imitation of Photoshop. I made a quick attempt to do what you are asking and it seemed to work nicely.
It’s pretty simple, although the tool is really lacking in that it doesn’t provide a ruler so that you can measure things up (so I used XScope, a little tool I have).
So I’m not saying this is ideal, but you wanted something free!
What I did was
Create a new image > Presets 1024 x 768 > then set height to about 40px
Choose a foreground color (for the main color of the image) > choose paint bucket tool > click on image to fill it with that color (if other than white)
Change the background color (colored boxes on the bottom of the left toolbox) to to the color you want for the left column > Choose drawing tool > drag a box over the left and of the image.
Do same process for the right end of the image.
Then maybe crop the image to a better size (say 10px high).
Then save.
This is the first time I’ve used pixelr, so if those instructions aren’t clear, just play around and find something that works for you. 
Or buy a cheaper alternative to Photoshop. There are various ones out there that are very cheap. E.g. http://store.smithmicro.com/productDetails.aspx?pid=2404
Care to share how you actually created the image thingy?! 
TomTees