Using smoke brushes in combination with a gradient map can create an effect that looks very much like fire.
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First you will need a black background, where you will draw your smoke with white color on:
Personally I like to use Solid Color Layers, and draw the smoke on the mask of the layer. If I want to change something later, I will be more flexible.
Next we will create a gradient map to color our smoke like fire. In the below sections you will find how it is done for Photoshop, Affinity Photo and Gimp. For the following gradient I’m using the colors black, #ff2400, #ffc000 and white.
In photoshop, we will use “Menu >> Layer >> New Adjustment Layer >> Gradient Map”, using gradient like the one above. The result will look like this:
In Affinity Photo, go to Menu >> Layer >> New Adjustment Layer >> Gradient Map Adjustment”. Use a fire colored gradient to color your smoke.
In Gimp the process is a bit different. First you will need to have everything on a single layer. So paint your smoke directly on a layer filled with black:
Then select the gradient tool and use a fire like gradient. Gimp has a predefined gradient called “incandescent” which works quite well for our purpose:
Then use “Menu >> Color >> Map >> Gradient Map” to map the colors to your black and white layer.
Play around with the colors of the gradient to change the effect. Depending on the colors you choose and how you distribute them you can create different effects.