'Mana' is simply a count of energy you have generated from your permanents. If you tap a forest, you have added one green 'mana' to your 'mana pool', so you can spend that one green mana on a spell or ability.
If a card adds mana to your mana pool, you aren't searching for any card, there is no card 'called' mana, and the card does not mean you get to do anything with 'land' cards from hand or library. All it means is you've now got some extra mana to use that phase, exactly as if you'd tapped some land for it.
red
Most of the time : yes.
Black Lotus is an artifact with 0 casting cost. It has tap, sacrifice: add three mana of any color to your mana pool
Lands usually have no color, they are colorless, since they have no mana cost Lands usually have no color, they are colorless, since they have no mana cost
wish counters make you pay a certain amount of mana to reaveal the top card off your deck you may then play that card without paying its mana cost.
You're mixing 'land' and 'mana'. Your 'mana pool' is a count of how much mana you have generated from your resources. Imagine you tap a forest for mana - you now have 1 green mana in your 'mana pool'. Old versions of forests used to actually say that, but it's assumed everyone knows what Basic Land does, it taps for one unit of the pictured mana.So lands generate mana, but so do other cards, like Birds of Paradise, and Dawn's Reflection. They are not implying you fetch land cards from your deck, they are just adding mana to that mana pool, like Basic Land does.So imagine that forest is enchanted with Dawn's Reflection. When you tap the forest for mana, as well as that one green, you add two mana of any colour to your mana pool. That one land is effectively generating three mana, which you can then cast spells with.
Mana is magic. To get mana you need to collect 10 falling stars to create a mana star. Mana is used for magic items such as a flower of fire.
Yes they are. Regardless of the colour of mana they produce, a land is still a colourless permanent unless an external source is giving it a colour.
There are many flying walls that use white mana. Wall of Denial, Wall of Reverence, Sunweb, Wall of Shards, Angelic Wall, Wall of Swords, Ageless Sentinels.
You are not restricted by what colors you have in your deck in any Magic format except for Commander (EDH). In Commander, you cannot have cards in your deck that have a color identity different from the general you chose. The color identity of a card is that card's color plus whatever mana symbols are in that cards text box (except for those in parentheses).
Magic
it's not an elf but it's a rebel and it searches your library for an elf and puts it into play for three mana it's called Skyshroud Poacher