An image in Photoshop is color. Whether the image is in bright, vivid colors, various shades of gray, or even just black and white, it doesn't exist without color. At its heart, Photoshop is about assigning the correct color to each pixel.
Be aware that the term “bitmap” can have two different meanings. It can be used as a synonym for raster (“That is not vector artwork; it's a bitmap image.”). In this usage, it refers to an image consisting of pixels arranged in rows and columns. When talking about color, “bitmap” refers to black-and-white images. In Bitmap color mode (also called 1-bit color), each pixel is either black or white. There are no shades of gray or other colors.