If you're trying to open a command window and have it look a certain way, then you'll definitely have to change the syntax a bit. So, let's look at what you're doing first. I'm going to assume that the command line you've given us is literally what you're typing in the 'run' box. Let's break it down a bit,
1) cmd
If you enter this by itself, does a DOS command window open? If not, then you have some other issues. But for the sake of this question, I'll assume it works.
2) cmd [/C] [/T:fg]
Get rid of any and all square brackets [ ] .. those are used on the help screen to delimit the start and stop of the different command line options. They are not meant to be typed in.
3) cmd /C
This will execute whatever you want in a DOS command window, then immediately close the command window. That's why it's disappearing so quickly. If you want the DOS window to stay open then use,
cmd /K
4) /T:fg
The 'fg' are placeholder variables. 'f' means Foregroud, and 'g' means backGround. But 'fg' is not supposed to be typed in, it's just meant to show that there are two variables and their order. To actually use the /T option, then you'll need to look at the options on the COLOR command, use COLOR /? to see those.
So, for instance, if you wanted a blue background with white text, then you would enter
cmd /T:17
5) One last quirk, if you put the command and options in this order, you'll get an error.
cmd /K /T:17
The command options need to be switched around for this. I don't know why, has to do with how DOS processes the ":" symbol as a label or part of a drive name, or some such nonsense.
So the correct command with options to create a non-closing DOS window with a blue background and white text is:
cmd /T:17 /K
Finally) if you want your chosen colors to be persistent any time you open a dos window without have to type all those command options, then open a window with just
cmd
then, right click on the title bar of the DOS window and select "Properties". Make all the changes you want to many aspects of a DOS box, including background/font color, size, fonts, layout, etc. When you click "OK" to save, then you'll be prompted to save the properties .. choose "Save properties for future windows with the same title."
Hope this helps!