So far I had no luck trying to find the visible absorption spectrum of $\rm CO_2$ anywhere, all I get is the far infrared absorption spectrum and stuff like that.
If you just search "what color is liquid $\rm CO_2$" it simply says everywhere that "it's colorless" but... That's also said a lot about other liquids such as water, but we all know that's not true: https://en.wikipedia.org/wiki/Color_of_water#:~:text=While%20relatively%20small%20quantities%20of,and%20scattering%20of%20white%20light.
Water is not colorless at all, it just appears colorless in small quantities since it's a very transparent liquid and one can only appreciate its slightly blue coloration in very large quantities such like in pools, lakes, oceans, etc. Water is not just blue because of Rayleigh scattering and the sky's reflection, it is also and mainly blue because it does in fact absorb more red and green light than it absorbs blue light, just like any other blue thing.
That being said, I don't think liquid CO2 and other liquids such as liquid SO2, NH3, CH4, etc. are colorless as it's said and as they are described everywhere.
However, going back to just liquid $\rm CO_2$. I believe it just appears colorless in small quantities, but in very large quantities some color is perceivable exactly just like it happens with water. But I might be totally wrong since that belief is based on absolutely nothing, that's simply how I guess it is since I have absolutely no idea and it's simply hard to imagine a liquid that totally ignores visible light without absorbing even the slightest bit of it at all.
But If that is the reality and it doesn't absorb visible light at all, then that would mean deep liquid $\rm CO_2$ would just end up appearing slightly blue. Much less than water does though, it would be a much clearer liquid and you would need way more deepness for the blue coloration to appear because this coloration would only come from rayleight scattering. I think this is the same that happens with liquid N2 based on previous research... But there is pretty much almost no information at all about liquid N2's color too.
CO2 is such a common thing in this universe that a lot of exoplanets out there might have vast liquid $\rm CO_2$ ocean, seas or some lakes since the conditions needed for a planet to allow it are not that crazy. That's why Im interested about knowing the answer to this question. I'm simply very curious about it and I want to have an slight idea of what would an hypotetic exoplanet with $\rm CO_2$ oceans look like. Before anyone mentions that I should ask this question somewhere else like in"wordbuilding"... I already tried there without luck. I was told to ask the question here too to see if some of you have an answer to this simple yet so seemingly hard to answer questions due to how impossible it is to find anything about liquid's $\rm CO_2$ visible absorption spectrum over the internet. Nothing in Reddit, Wikipedia, Google, Quora... There's absolutely nothing, and if there is, I wonder where because I searched non stop everywhere.
Sorry if there's any grammar mistake, tried to avoid making one and I believe it's all right.
Had this question unanswered for a long time, would love to finally get an answer.