Heat transfer happens through 3 mechanisms, each of which is sufficient to make you uncomfortable:
- Conduction (solid to solid or liquid) -> you want centimeters or more of insulation, ideally some inertia, and an eye on your thermal bridges.
- Convection (gas movement) -> you want to keep an eye on ventilation or air leaks.
- Radiation (infrared, sunlight...) -> here's the domain of reflective thin film insulators, heat-absorbing paints, well-placed windows, and thoughtful usage of sun shades.
Solar tanks for hot water should be insulated (typically a stainless steel + PU foam sandwich), so whatever their color doesn't matter much to their performance. Thermal solar panels already have thermochromic coatings. Mixing the function of the tank and the panel only makes them more complex. Efficiency for thermal solar is not a big differentiator from one technology to another, so I suggest going for the cheapest/most widely available one in your particular market.
There are some solutions out there for thermal solar panels connected to radiators or radiant floor. It's what we call active solar (passive being when you just have big windows or the like, as in our straw-bale house). They generally have a lower water temperature than furnace-powered radiators (meaning larger radiators for the same heating power). The problem is when there's no sun. You can have either a back-up heating system, a well-insulated house with high inertia (outside insulation), or a multi-day hot water storage tank (a few cubic meters ; again, it will require insulation). Looking up historical weather data on the Internet for the past few years would help you assess how much storage you'd need.
Thermochromic paint is tricky, because if it's inside the insulation layer, then it's going to dissipate the heat it collects inside whether it occludes or not. I'm not aware of any solution that would work in that scenario. However, you can lookup the Trombe wall technique that was developed in the 1970es: a small green house made from a glass panel installed a few cm away from your sun-facing wall, and vents that can direct the convection flow either inside or outside the house to regulate the inside temperature. They're obsolete for new constructions, but can make sense in a low-tech renovation project, especially when the weather is not extreme. Another solution is to leverage outside sunshades in front of windows, a veranda, or even a large dark wall with no insulation (playing on the inertia here). You probably want to run the math before you commit to renovation, especially if you are richer in time than in money.
I'm not sure about photochromic windows. The devil is in the details. Electrochromic is probably a no-go, because its power consumption is high (~100W for a large window). Photochromic is going to heat your glass, so if it's on the outside pane of high-performance double-glazed windows, maybe it's OK ; but you'd want to look into the absorption spectra (light, infra-red...), and having it turned on by the UV exposure robs you of the benefit of actual regulation (sunny winter days are typically the coldest). In any case, it's likely to be more high-tech, and as such more expensive (but not necessarily more efficient) than just hanging a piece of cloth outside your window, that you can raise or lower depending on the house temperature. Just as windows-with-solar-panels-inside, it's a blingy high-tech solution that fits well some prestige architecture projects, but not necessary home renovation plan.