(Fig.09) I decided to leave the background for now, and started on the monster. I made another new layer, under the line layer, and painted in the approximate shape of the monster using a darker colour for the head (which is closest), and a faded colour towards the tail to create a sense of depth. I often find that it's important to choose the correct colour at this stage because this base colour will show through a lot when you continue to paint the details. I didn't really use any colour rule for this piece - just what I thought would work.
Highlights & Shadows
(Fig.10) I firstly decided to make the ground darker. I really had no idea how the ground would look if it was seen under lava, but I felt it should be darker. I also played with the Brightness/Contrast and Curves to tweak the colour a little, giving the background a darker, saturated tone to represent the intense heat of lava. As for the creature, I started painting in shadows to form the basic shape of the creature. I then added colour variation to the highlights; some green to contrast with the orange. I played around with the colours without really having anything specific in mind.
Painting Over The Lines 1
(Fig.11) I can never wait for this step, but I always try to hold myself back from rushing into it until I feel that I have a decent idea of the right colours, and so on. I basically made a layer over the lines, and also started working in some details to the background. I think that many artists work differently with regards to how they paint over the lines, and it's a difficult to explain exactly how I do this myself. I used the Colour Picker a lot, with the Alt key, and tried to sculpt and refine the shapes and details.
Painting Over The Lines 2
(Fig.12) I continued refining and adding details, and so on. Because the tail is further back, it is less detailed and the contrast of it is lower. I tended to work backwards at times, and I got rid of details for the parts that were further away to bring focus to the head. I also got rid of the rough pencil lines that were sticking out, and repositioned one of the feet. I spent some time painting over the actual lines of the line art, mainly because I didn't want the creature to have a black outline around it, and because the tail part goes further into the distance the line shouldn't be the same colour as the head.
(Fig.13) Here I started adding smaller details, such as cracks, and tentacle bits on the spine. I decided that I wanted some glowing parts, which are always fun to add, and I thought it would work well fading off as it reached the tail. It's often the smallest details that really help to bring out the piece. I added highlights of red to the mouth, added some red/pink to the head, some jagged edges to the bottom rim of the jaw, and so on. I also got rid of the background lines at this stage as I didn't really need them anymore. Before making the final touches to the details, I saved the file, flattened the layer, and increased the contrast and saturation of the whole piece. It didn't quite feel hot enough before, for me. I sat there trying to think of more details to throw in, because I really wanted to push the character and make it better, and I finally decided to add flames coming out of the spine, and more rocky spikes on the jaw.
Background & Details
(Fig.14) I thought I was basically done with the creature at this point. However, the background didn't have enough mood, and so I added heat waves coming out of the cracks. I continued adding more details and cleaned up some of the brush strokes in the background. I also got rid of the one volcano on the side, because I thought it was a bit silly only having one.
(Fig.15) I still didn't think it felt hot enough for lava, even at this stage, and so I further increased the intensity of the colour by adding some flames coming from the mouth, and so on. I also cropped the piece slightly as there was too much empty space. The final step in a lot of my work recently is playing around with another round of textures, usually at a low opacity, to achieve a more painterly look. This is usually done by simply throwing in a layer, setting it to Overlay mode, erasing some parts, and painting over others. For this piece I also flattened the layer, duplicated it, and threw in a Photoshop canvas texture filter (yes, a Photoshop filter). I erased parts here and there using custom brushes. I tried to aim for a painterly look, and looked at many real paintings as research. Basically, in places where artists haven't used use a lot of paint, the canvas texture faintly shows through. This step is either a hit or miss. Sometimes Photoshop filters are dangerous and can look "cheesy".
Usually, when I save a small JPG for submission or for a website, I use the Photoshop Sharpen filter and fade it to around 40-50%, which brings out some of the brush strokes and detail.
That's all! I hope it has helped.