When I started making minicomics in the mid-90's creating comics in color was out of the question. Photocopying being what it is/was, color was way too expensive, so black and white was an economic necessity. Luckily I had a number of examples at the time of great black and white comics, like Cerebus or Love and Rockets. Because of this, I never really though about using color in my comics for years. It wasn't until I started a webcomic that color become an option. Publishing on the web provides no printing barriers to using as many colors as you want, so working in black and white becomes a stylistic choice rather than an economic necessity. Three years later and I'm still thinking about color. My current webcomic (Things Change) is structured as a set of short stories, and part of the reason for this is so I can experiment, including in using color. There is no better way to speak of comics than with words and pictures, so here are some pictures (mostly from others) and words (from me) on how different artists use color with an emphasis on how one can change colors from one panel or page to the next.
You don't see it often but, this panel from Frank Miller's That Yellow Bastard (Dark Horse) is as decent an example as any of the use of spot colors. Most of the book is in black and white (mostly black in this book) but he uses a spot yellow at certain points for the eponymous character (or his blood in this case). This little bit of color can really punch up a panel/page and adds an almost otherworldly significance to whatever is colored by being outside the realm of the normal black and white. You see this occasionally in Dinosaur Comics too (the devil's words are in red text).
Sammy Harkham uses a black line with an multi-tonal orange wash in this story from Drawn & Quarterly Showcase. In these panels the color is mostly used to create atmosphere: from the sharp contrast of nighttime tv watching to the sun breaking into a dim, curtained room. The wash provides its own visual dynamism in its varying uneven tones. One of the great things about using only one color at a tome is that it is easy to change that color during the story and have a great effect on the tone of the images.
Adam Sacks does this in Salmon Doubts (Alternative Comics), using only two colors at a time, but by switching those two colors at different points in the narrative he changes the mood and setting.
In this wonderful panel from Gray Horses (Oni), Hope Larson, drawing with two brownish colors, uses her tones to reclaim white as a color. By using the browns in the majority of the panel, white, as the minority space, becomes more positive space then the negative space it usually is (color margins and gutters will get the same effect).
Frank Santoro uses color as often for composition as for any realistic effect. Throughout Incanto (Picturebox) orange and blue are used to structure pages and panels, as in this two page spread where the characters are alternately orange then blue in contrast with the backgrounds making for a beautifully balanced composition.
Nick Bertozzi uses color in a similar way to both Sacks and Santoro. He uses two colors at a time, shifting them throughout The Salon (St. Martin's) to change the mood and setting, but by putting characters in one color and backgrounds/settings in another he can control the composition of his page in more ways than with just line. The second image here, with its darker, cooler colors that are rather similar to each other evoke an evening where lights are dim and colors blend.
Gene Colan mixes realist linework with an almost monochromatic palette in this nighttime sequence, followed by a wider range of colors when the woman has gone into her lighted room. (From Marvel Romance)
In the same volume, Jim Steranko, put pastel colored figures in front of a high contrast black and white background. The colors seem to get richer for the figures closer to the viewer.
Instead of just filling in black lines with color, in Cold Heat (Picturebox), Frank Santoro uses color for lines, tones, composition, and expression. Note the change of color in the woman from one panel to the next and the way the background of the one panel is drawn in pink while the foreground is drawn in blue.
In my own work, I used color two differentiate two time periods in a story. Throughout the short story one color represents the present and one represents the past allowing for an easy differentiation.
Paul Hornschemeier uses different color schemes for the various comics within the comic of his The Three Paradoxes (Fantagraphics). In the case of the two panels above, he colors the margins and gutters of one of the stories to give it that patina of age, and in doing so changes how the colors look by virtue of the white/off-white reacting with the other colors. The first transition is also a great example of shifting a color scheme to shift lighting.
I hope that spurs the imagination a bit on using colors or "reading" the use of colors. And yes, I realize these are all print comics (except my panel), but I couldn't come up with any examples in webcomics as most webcomics seem to be in black and white or stick to a rather uniform color palette.
(All images are copyright their respective artists, except the Colan and Steranko pieces which are no doubt owned by Marvel.)