VBA - How to create a user defined function?

It's probably pretty easy question, but I am not familiar with the syntax of VBA, so I need some help.

I would like to have a function called vcolor(), where it uses a number to determine a cell's color.

If the number is <= 30, it's red. 'cell.Interior.ColorIndex = 3'
If the number is >30 and <= 80 it's yellow. 'cell.Interior.ColorIndex = 7'
If the number is >80, it's green. 'cell.Interior.ColorIndex = 4'

I would like it to return the same number, and simply change the cell's color.

Use the above strings to change the color (I believe that's correct), I just don't know how to add them into it correctly due to syntax, thanks.

UDFs, when called by a worksheet formula, cannot change a cell's color. All they can do is return a value.

You can do it via conditional formatting:

http://www.contextures.com/xlCondFormat01.html

would conditional formatting have the colors change, as the contents of said cell changes?

that's what I'm looking for.

I also could have sworn I saw somewhere you could do this with UDFs, trying to go through my browsing history now.

Edit:

The conditional formatting seems to work how I want, although there is a small limitation.

I can only have it work as

<30 red
>= 30 and <=80 yellow
>80 green

That's fairly close to what I wanted, might have to live with it.

In versions of Excel later than 2003, you can have as many conditions as you like.

Yes, you're missing a bunch. All of those things are possible.