The red mark originally signified that a woman was married, but over time it became fashion, more a mark that you were Hindu. Now everyone wears them, regardless of age or marital status. (In some regions, men wear them, too.)
Size, shape, and placement on the forehead might once have conveyed geographic or socio-economic information, but now it’s a matter of taste. My great-grandmother-in-law drew hers as a horizontal line. People from south India might use a vertical line. The dot is common, but the possibilities are endless. Bindi should generally be red, but they come in all colors and levels of decoration, to match your sari/salwar/lengha and the formality of your event.
The bindi itself has also evolved over time. For thousands of years, people used turmeric powder, which turns bright red in an alkaline environment. In the 60s, red liquid was in vogue. Nowadays, everyone uses stickers.
If you’re in India, a packet of plain bindi costs between 5 rupees (about 10 cents); a really fancy one might cost you as much as a dollar.