Updated! Flash effects are now in.
The way colors work throughout the whole program have been pretty much completely overhauled, color selectors now use RGBA numeric selectors instead of a text field, which is more consistent overall. You've still got that HTML button to select a specific color as well.
Took a bit of testing to get a good balance between making it simple to do basic stuff and also being capable of more complicated effects, but I think it's come out pretty good. You can add a simple color flash event, a looping color flash, a 2-color blink, add color flashes to hitboxes (you'll notice that all hitboxes now create a small white flash by default), link flashes to sprites (which causes the flash to go away when the sprite is destroyed, allowing you to apply a continuous graphic flash along with a regular status effect), adjust the timing of the flash "wave", and even add a cubic-bezier curve to customize the flash effect however you like, if you're into that kind of thing. Flash effects can be applied to sprites or over the entire _room. (Some of the continuous flash timing might still be a little off, but its intended functionality is pretty self-explanatory in the tooltips. I'll debug it later.)
Flash effects are cumulative. A single sprite can have multiple flash effects at once.
You can also adjust the basic color mask of a sprite by using the sprite._color variable. Colors are objects, they can have their _r, _g, _b, and _a (_a is alpha/opacity) values set through a function. So you can make a sprite's color more or less opaque by changing sprite._color._a; this serves the same role that _flashlevel did before. (_flashlevel should still work but it may be depreciated in the future since it's redundant.) _flashcolor no longer works.
I think most existing colorization effects should be retained, except in the case where a color was set inside a function, but that's okay because the new flash events are just better overall.