For under $15 you could add a whole selection of colors to your kit. Color for your backgrounds or for your subject lighting. They're called flash gels. Flash gels are simply colored, transparency slides that you put on or over your light source to change the output color. In the sample images I took, all I was changing was the color of the flash gel and it completely changed each background. A lot easier and cheaper than buying a bunch of backdrops for your studio and all I had to do was fire them remotely. They can also be great for matching lighting color when you have mixed white balances in a single shot.
I personally use a simple set of Rosco 55 slide collection , but there are truly a ton of options when it comes to getting flash gels. You can get them for different models of strobes (including speedlights) or for constant lighting. Their pricing can range from $10 to almost $100 or more for kits. However, I would suggest that the simple kit I have for my speedlights will take you leaps and bounds.
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