To begin drawing fire effects, lets take a look first at
how to draw a fire:
The general shape of a flame is tear-shaped as shown on the candle to the far left. Iconically, this is the simplist form of a flame but you may progress in detail depending on your personal drawing style.
To go to the next level of detail, you just add smaller tear-shapes at the top of the simple flame shape and combine them to form the next representational level of a flame.
For those interested in the realistis style, you simply add more of the tear-shapes all around the basic flame shape. Make sure you 1) vary the size of each tear for variety, 2) curve your lines as no standing flame is straight, 3) make sure each rising flame has a general upwards motion, and 4) add extra detials like embers and a "second" fire layer which is lighter in color than the outer fire edges. Remember, depending on the situation, these may change. For example, if fire is churned out through a blow-torch or similar way - it will come out straight because of the force pushing it outwards.
To draw embers, there are basically four shapes:
1) Triangle - typical ember representation
2) Diamond - usually bigger than the traingle embers
3) Dots - for the minute, hard-to-see embers flying in the air
4) Crescent - for the burst of fire that basically looks like it's jumping and swirling in the air giving it that curved, crescent look.
Embers may take on its basic form but a few jagged edges are usually added for the extra distinction.
Let's take a closer look at how a fire spreads out... Here to the left, notice the marked arrows. These show you the general direction of fire if the source is from the ground and isn't contained or beside anything.
If the flame was beside a building, let's say, only half of the shape would be drawn:
Building --> =||/_ <-- 1/2 Flame
For non-ASCII literates: We starts with 7 arrows for a full flame but beside a building, it would only need 4 arrows starting with the bottom arrow.
For fire exploding in mid-air, you would have a circular fire using two of the flames shown above - one atop the other.
To draw fires in a line, you basically just draw several basic flame shapes in a row and add in details. This is a great backdrop for characters with a lot of anger in your manga. It also comes in handy if you're doing a flame-following-the-fuel-scene.
All in all, what you really need to remember when drawing
fire is 1) how detailed you want to draw, 2) if you'll add
embers, and 3) what direction the fire will go. If you're
having trouble drawing the fire correctly, just conceptualize
how you will draw your fire by using basic shapes first
and then going from there.
If you're really stuck, make sure to check out these flame studies put together by ZionSavior! Includes: Candlefire, gunfire, welding fire, rocket fire, campfire, jet fire, flame thrower, and dragon fire.
Notice how the same fire can be used for different purposes such as the flame thrower fire can also be used as a dragon's fire breath.
As for the colors of fire, they are usually orange to red and light blue to dark blue/purple. You can expreiment with the colors as it suits you so feel free to go crazy.