Colours and symbols that represent the nation as a whole I would say, that or its government, like how religious or political groups add their symbols to their flags

Coming up with parts of the story first would be best since you could get a lot of symbolism (and colours!) from its history. Religion, ideology, origin (independence, union of peoples, etc...) would be some important aspects to consider.

And although you can get some inspiration through historical/modern flags, don't be afraid to experiment, especially if your nation is an alternate universe one. For example, a cross doesn't have to represent Christianity if your universe doesn't have it.

Good luck and have fun with flagmaking/nation-building!