Focus on looking for reddish, rust-colored areas if Iron is a priority. Iron is fairly easy to identify due to being so common and in such contrast to the grey of the surrounding stone. Typically a matte midtone grey, the appearance of stone can vary a bit between brown and black. However, once you have identified the vein you wish to obtain, it can often be more effective to rely on sight to guide your mining, and so a Spotlight is almost always useful for this purpose, to more easily see where one vein ends and another begins. This will put the name of the ore on the HUD in approximately the center of the ore patch.
It is for this reason that an Ore Detector is always highly recommended for the initial identification process. Identifying ore patches purely by sight can be frustrating, especially if there are two similar ores next to one another such as Platinum and Silicon. Using a Hand Drill or ship-mounted Drill will destroy the rock and release the Ores as objects in the world to be collected.
Ore is a basic resource item, usually obtained by mining Asteroids.