Question: I'm building a piece of software which will have a filtering system that involves multiple flags. The complication is that each flag has three possible states:

1. On
2. Off
3. N/A (i.e. It can't be applied, for whatever reason)

Here's my current plan:

![enter image description here](https://i.stack.imgur.com/xPbFC.png)

So the "Size" and "Weight" flags are "on", "Height" is off and "Lid Width" is N/A (None of the products actually have a lid). Clicking each box toggles the flag, unless it is N/A.

However this method has it's limitations: One of which is that it relies on colour, preventing use by colour-blind users. I could use checkboxes instead, but they take longer to read and absorb when there are a lot visible at once.

N.B. The software will *only* be used by a small selection of experts for many hours a day. So it is less important for it to be easy to learn, and more important for it to be quick to use and visually clear what's going on at all times.

Answer: 
Good answers here, but they don't mention the common name for this UI element...

These are called **"[tri-state checkboxes](http://en.wikipedia.org/wiki/Checkbox#Tri-state_checkbox)"** (wikipedia), and are often used to show a "mixed" or "other" state in toggle switches.

![enter image description here](https://i.stack.imgur.com/5znJJ.png)
![enter image description here](https://i.stack.imgur.com/gnmqw.png)
![enter image description here](https://i.stack.imgur.com/bqTXP.png)

[**more examples here...**](https://www.google.com/search?q=tri-state%20checkbox&source=lnms&tbm=isch)
Red color is eye-attractive although it is for off state. Besides it could be not pleasant while long observation. 

I suggest other styles for the states distinction. Dots allow quick eye-jumps and have some meaning (on-off).  

![enter image description here](https://i.stack.imgur.com/Ss96M.png)