Let's say I'm given an image of a flat colored icon, which is a 512x512 PNG black icon. Is there a software to quickly and simply generate e.g. a 36x36 (arbitrary) red (or any color) version of the same icon? Such like macOS' Preview, which is quick to open and from there is straight forward. I find it overkill to fire up GIMP or Photoshop for what feels like such a simple task. Preferably on macOS.
In your comments you mention that you have yet to find "a simple way to change the entire image's color while keeping its alpha" in GIMP.
However that is fairly simple task.
In GIMP, open the image, make sure the Image mode is RGB by clicking Image > Mode and that check that RGB is selected.
Then click Edit > Colorize, then move the hue, saturation or lightness sliders to colour the icon as you wish, or alternatively use the colour selector. It will not affect the alpha channel at all.
Here's an example where I turn a monochrome (black and white) icon into a bright red icon.
Resizing images is also simple. Click Image > Scale Image, and change the size. Remember that scaling raster images will decrease their quality. If you find yourself scaling images often, probably better to look into using vector image editing software such as Inkscape (which is also free and Open Source), or Illustrator (not free). Vectors can be resized smaller or larger without degradation in quality.
Note: If you are using Photoshop, the same is also possible, but the colorize option is in the Hue-Saturation adjustment dialog, and image sizing is under Image > Image Size.
You can do that at least two ways with Photoshop.
Create an Action in Photoshop for the change.
Then in File>Automate you can either use "Batch" or "Create a Droplet". In these menus, you specify how you want them to processed, meaning do you want them saved over the old files? or duplicates made before processing and then saved not another folder/directory?
Batch will let you control the processing while Photoshop is open. A Droplet is a special mini-app it creates that you can just drag and drop your files onto and the Action will be performed on all of them.
Depending on the variety, you might need to create a few different Batches or Droplets for the different steps or final versions you need.