https://i.imgur.com/dTiXRIB.gif
Is there a tool that allows me to do a colorshift effect like in this gif?
The effect where the cape goes from black to red constantly in a fluid way.
I tried searching online, but I only find how to make glow effects, or how to shade light, not an effect like in the gif above.
I use a script to create a gradient that spans all frames:
Then I use a script to multiply the alpha of two layers, which shapes the blocks of color:
This is the result:

Obviously, were this more than a quick demo, I’d want to isolate the cape’s inner lining to its own layer. If I wanted the red pulsing to be more frequent, I’d add more red and black stops to the gradient.
[Edit: For more nuance, where the shading on the original cape is reflected in the red tint, I’d use a custom blend instead of a layer mask:

]
Alucard sprite sheet source: The Spriters Resource - Full Sheet View - Castlevania: Symphony of the Night - Alucard
The sotn decomp project might give you a more exact idea of how the effect was created for this case, depending on if the community has gotten to that part of the code.