From cold to warm
First of all I opened the original image in PhotoShop.
I then went to Image > Adjustments > Invert.
The next step was to go toImage > Adjustments > Threshold and entered 125 into the levels.
I created a new layer and set the blend mode to Linear Light. I selected the Gradient Tool and with the foreground colour set to a bright orange (ff6600) and the background colour set to white I applied the gradient from the top left corner up to the bottom right. Repeat this process but change the direction. This is the final result:
