White and Black tank

Let's make an experiment : let's take 2 identical cans/tanks one black painted and the other one white painted. At the beginning, they are filled with water of which the temperature displays 25°C/ 77°F.

Let's exposure them to the sun with the same orientation and let's wait.

At about half an hour later, the temperatures displayed are different in each can/tank :

the white one displays 28°C/82.4°F and the black one displays 32°C/89.6°F

Relevant wiki: Blackbody Radiation

Since every black body absorbs all the colors in the visible spectrum, during day it will get heated up faster than the white tank, as the tank painted in white reflects all light falling on it.

During night the black colored tank radiates all the energy it had absorbed during the day making the water inside it cold. So during night the water inside the white tank is warmer compared to the water in the black colored tank.

A white tank will primarily heat up via conduction with warmer air around it during the day. The black tank however, since it absorbs more radiation, will also acquire much of it's heat via radiation.

The black tank will be prohibited from having a temperature exceeding that by much of air temperature, because if it does, it will simply have that heat conducted away from it, so by the end of the day, both tanks would be expected to have the same temperature as the air. This ensures they will both be at a similar temperature throughout the night.

But since the black tank is receiving more power during the day, it will heat up about as quickly as the air, while the white tank will heat up with a noticeable lag from the air temperature. The white tank heats slower during the day, and is thus more effective.

While a black tank will absorb and emit energy more readily, therefore heating up faster in the day and cooling faster at night than a white tank, making white the colour of choice if our only consideration is water temperature, there is a big disadvantage to the use of white tanks! Water tanks are most often made of plastic these days, and white tanks transmit more light, encouraging higher levels of bacterial and algal growth, rendering the water less safe for domestic use. For this reason, white water tanks are not commonly used on rooftops.

For that matter, having a black water tank on the roof in a hot country gives you a free supply of hot water, reducing energy bills.

We know that white colour reflects all the seven colours in light(coming directly from sun) thats why its absorbtion is minimum. While black light absorb all the seven colours due to which it absorb maximum energy. So water in white coloured tank will absorb and emit heat in large interval of time as compared to black one.

shouldn't the black dissipate more heat at night but absorb it during the day? and the reverse for white, so if you want water in the day the white will be cooler?

Black color looks black to our eyes because it absorbs all colors.

White color looks white because it reflects all colors.

Here by color the only thing available is sun rays, so black tank will absorb all radiations from sun rays and white tank reflect them all.

Hence, white tank will be cooler during day.

In modern elementary school, we learn that darker colors absorb heat, while lighter colors reflect it. Therefore, the lighter tank should reflect more heat and therefore the water should be colder.

The tank colored white will reflect some of the sun's rays, causing it to stay cooler for longer during the day, whereas the tank colored black will absorb more of the sun's rays, causing it to heat up faster during the day. This makes the white color preferable during the day. If the white tank never got sufficiently warm, however, this would be bad for the nighttime, when we need warmer water. So it is relevant that the location gets "very hot in the afternoon" and "very cold at night." This means that we may assume that even though the white tank would warm more slowly during the day, it will still approach a comparably warm temperature so that, come nighttime when the colors have less effect on the heat dissipation, the two tanks will have comparably warm water cooling at comparable rates.

Well because black attracts so much heat and white reflects, if is most often going to be that black has more heat than the white tank. Also it may depend on the kind of container your putting it in.