Currently AFAIK you can either color a liquid or remove coloration entirely but there isn't really a way to set how transparent the liquid is. i.e somewhere between fully transparent and deeply colored to the point of being opaque.

Have you tried passing alpha in the Color creation?

Not yet, there's no mention of it in the fluid creation or color sections of the wiki and I haven't found the section that talks about alpha as a parameter so as soon as I figure out how to do that I will.

https://css-tricks.com/8-digit-hex-codes/