OBSERVATION PT. 2 - LIGHTING
All imagery is formed by light - light bouncing off of surfaces and hitting our eyes.
Much of what I'm about to say may seem obvious, but it needs to be considered briefly when creating art realistically from the imagination.
Think of the 2d art surface as a view of a 3d space. How do we represent light hitting and bouncing off objects? Shadows are formed by opaque objects blocking a light source. Consider the direction of light sources in your scene and how that will create shadows on the surfaces opposite the light sources. Consider the fact that the color of the light is the color that'll be absent in that light's shadow. Thus, warm lights will cast cool shadows and vice-versa. Consider where the lights are in your scene and the forms that block them. Also consider reflections. Water reflects the colors of the sky, which is why it is usually bluish, but in cases of stagnant water it may be more brown or green depending on what is mixed into it. There are, in my mind, three layers to water - the bottom (what is behind the water) the volume (the tint of the mud, algae, or other particles in the water) and the reflections on the surface. The surface might be flat or rippled; if a reflective surface is curved or in some other shape, just remember that all the parts of the surface will reflect whatever is 'bounced' from the surface to the viewer's eye. Try mentally reversing this if it helps - imagine a narrow beam of light moving from your eye to the '3d' surface. Now imagine it 'bouncing' off the surface to a point somewhere else in the scene. The place it hits is the place which we'd expect to be reflected. In some cases, this is not even in the scene itself - mirrored surfaces offer a chance to show areas outside the frame in the form of a reflection!
Note that moonlight is cool (bluish) and firelight an orangish-red, flourescent light often a harsh yellow-green or pure white, and halogen incandescent lights a warm yellowish white. Also note that partially opaque surfaces like stained glass will project the color of the material the light is moving through onto objects the light hits through that material. Finally, note that there's a little bit of bounce light on most surfaces - light not from a light source but bounced from a light source off an object onto that surface. This bounce light will tint that surface slightly with both the color of the initial light source and the color of the object it was bounced off of first.
COLOR IS RELATIVE - remember that colors are seen in terms of comparison with surrounding colors. You've probably seen the old optical illusion which surrounds two gray boxes, one with white and one with black, and noticed how they look like different shades of gray. Keep this in mind when trying to make a color pop out and look really dramatic and vivid
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