You've recently bought or you're considering buying one of my asset packs. However, the colors aren't ideal for you, and you're not very handy in graphics software. Here's a quick, easy to follow, and free way to tweak those colors. It doesn't require any special knowledge or software downloads.
Step 1: Go to photopea.com, click "Open From Computer", and choose the image you want to adjust.
Step 2: From the menu at the top, go to Image > Adjustments > Hue/Saturation (or press CTRL + U). You'll most likely want to check the "Colorize" checkbox before playing with the sliders, but explore the settings until you get a color you're happy with. When you're done, click "OK" in the Hue/Saturation dialogue box.
Step 3: From the menu at the top, go to File > Export as > PNG. When you click "Save" in the "Save for web" dialogue box that pops up, it will automatically save the PNG to the default browser download location. You'll probably want to change the name of the PNG and move it to your assets folder.
That's pretty much it. When you import graphics with multiple colors, the "Colorize" option will apply color to the entire layer. It's best to import images and adjust each layer separately to get the most control over your colors. The goal is to give you a predetermined set of assets with harmonious/unified color choices to quickly get your games and prototypes looking professional. However, this is a fast and free way to tweak colors, and all you need is a browser. You could also use Photopea to add text, icons, and illustrations to your assets.