There are several methods to convert black and white photos to color photos. One way is to use Solid Color adjustment layers in photo editing software such as Photoshop. This involves adding a Solid Color adjustment layer for every aspect of the photo you want to paint and changing the blending mode to Color so the detail of the original image shows through the paint. Then, invert the layer mask to black and use the brush to paint over the part of the image you want to colorize.
Another method is to use online tools such as GIMP, Fotor, Image Colorizer, and Algorithmia. These tools allow you to upload a black and white image, convert it to the appropriate color mode, and then use various features to manually add color to the photo. Some of these online tools use Deep Learning technology to automatically analyze and apply colors to the photo.
It's worth noting that colorizing black and white photos can be a great way to bring life to old photos and make them more visually appealing. However, it's important to consider the historical significance of the photo and the potential impact that colorizing it may have on its authenticity.