Not definitive, but as mentioned on the wiki

Interestingly, there is a recurring pattern of colors in the upgrade selection menu and the Class Tree. The first tank will always have a light blue background. The second will be green. The third will be red. The fourth will be yellow. The fifth will be dark blue, and the sixth will be purple (This order can be found in tank's add stats buttons which are color-reversed). Regardless of speculation, colors do not seem to represent anything other than a simple order pattern to more easily discern tanks apart when quickly selecting an upgrade

Note that Tank->Twin is light blue, and adds an forward extra barrel, while Machine Gun->Gunner has a similar effect, but in a different color.

It is interesting that tanks which appear in multiple paths (Twin Flank and Quad Tank) have the same color both times, but they have been placed in the same order to keep the pattern.

Image below for observing the tank pattern.

These are (build) points. Each point can increase one stat by one.

You only get 33 of them, and you get them as you level up. You get your first once you are past level 2. Then, you keep getting one for each level passed until level 28. After that, you get upgrade points at levels 30, 33, 36, 39, 42 and 45.

You can upgrade a stat by clicking the colored plus button if you have enough points to do that. They should be easy to find.

Otherwise, each plus button will be grayed out and will also slide out of view automatically. If you move your mouse make to the corner, it will reappear.

You can tell how many points you can spend by looking at the number to the top-right of the Health Regen plus button.

Special cases can be made for having 1 and 0 points:

• For one point, there will be no number, but there will be colored plus buttons.
• For zero points, there will be no number and there will be gray plus buttons. Like said before, everything will slide out when you have 0 points to spend.

Note: The mobile version of Diep.io refers to these as skill points, but the web version doesn't refer to them as anything at all. This tutorial simply calls them points.

Builds

Builds are the configuration of points. You might find something such as 2/2/3/5/7/7/5/2. Each section relates to one of the stats you can spend points on. Here is the points image, but there are arrows pointing from each part of the build to its respective stat in the points upgrade corner.

Even better, we could put the build vertically and each part would match up with each stat.

Going back to our example, we would put 2 points in Health Regen, 2 in Max Health, 3 in Body Damage, and so on. Here's the finished build.

Stat Descriptions

Now that we have gone through using points, I will discuss stats themselves.

• Health Regen is exactly what it sounds like. The more points you spend on this stat, the faster it takes to start fast-regening.
• Max Health is also exactly what it sounds like. It will take longer to kill you, since you have more maximum health. Think of it as finding a heart container from the Legend of Zelda.
• Body Damage is quite interesting. When your tank runs into an object, it will lose health and so will the object. This stat decreases the amount of health lost and increases the amount of damage done to the object. Sometimes this stat can be better than Max Health.
• Bullet Speed, similar to all the other stats, is true to its name. If you upgrade this stat, your bullets will move faster.
• Bullet Penetration, often called Bullet Pen, increases the length in which a bullet survives. Bullets decay after flying a certain amount of time or passing through objects. This, used in conjunction with Bullet Damage, can make a strong bullet.
• Bullet Damage is what you think it is. The bullet deals more damage, similar to if the bullet had more Body Damage.
• Reload means the more of this stat, the more bullets come out per second.
• Movement Speed is the speed you can move at.

Keybinds

When in an intense situation, you can't reach over to press the plus button. Instead you can use keybinds, which do the same exact thing as pressing the plus button. The keybinds are written to the left of the plus buttons.

Pressing M plus a stat number queues stat points into that stat once when they are available.

You are able to also use U to queue one point at a time. The March 9th, 2017 changelog describes the new behavior effectively:

• You can now queue stat upgrades by holding U then pressing the number keys
  • Pressing M will queue upgrades until the selected stat is full
  • Pressing U and releasing it resets the queue
  • Dying also resets it

In the below screenshot, the player has maxed out Bullet Penetration and Bullet Damage, and is currently maxing out Reload.

Points vs. Build Points

The build points, although referred to as points in this answer, are different from points, a nickname given to score. Build points are the ones showcased in this answer, while points are the number that appears below your name in Diep.io, seen below.

These regular points start at 0 and can go up to 1 million and above, while build points have a limit of 33 during a game.

Points indirectly influence build points, as points dictate levels, which dictate build points.