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Commander or edh is a format that, amongst other things places restrictions on the colours of the cards you play in the deck. See this faq.

A card's color identity is its colour plus the color of any mana symbols in the card's rules text. A card's color identity is established before the game begins, and cannot be changed by game effects. Cards in a deck may not have any colors in their color identity which are not shared with the Commander of the deck. (The identity of each card in the deck must be a subset of the Commander's)

But if I have a card like flash conscription which has a rider on which colour of mana was used, can I put that into a deck that doesn't have that colour identity, and can I pay for it with something like a treasure token or some other effect that can make any colour mana?

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As the rule states:

A card's color identity is its colour plus the color of any mana symbols in the card's rules text.

Flash Conscription's rules text contains a white mana symbol, therefore it it's colour identity is Red/White.

The only time this does not apply is when that symbol is included in reminder text e.g. Extort like on Crypt Ghast

it should also be noted:

903.4c The back face of a double-faced card (see rule 711) is included when determining a card's color identity.

so Elbrus, the Binding Blade / Withengar Unbound from is classed as a black card because the back face has the black colour identifier on it's type line.

Also split cards like those with fuse mechanic or aftermath also take the colour identity from both halves combined.

As for generating mana outside of your commanders colour identity, this is fine and allowed in commander rules, as of January 2016

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