Tokens that aren't a copy of something else don't have a mana cost. Anything without a mana cost normally has a mana value of 0.

You can choose any value for X as you cast Engineered Explosives. The value chosen for X doesn't directly affect the number of charge counters Engineered Explosives enters the battlefield with, but it does let you pay more mana and thus spend more colors of mana to cast it.

If a permanent has {X} in its mana cost, X is considered to be 0.

Colorless mana won't give Engineered Explosives another charge counter. Colorless is not a color.