Well, apparently they were back then and still are today for people of all ages.
According to Wikipedia: A coloring book (or colouring book) is a type of book containing line art for a reader to add color using crayons, colored pencils, marker pens, paint or other artistic media. Coloring books are generally used by children, though coloring books for adults are also available. They cover a wide range of subjects, from simple stories for children to intricate or abstract designs for adults. The golden age of coloring books is considered to be the 1960s.
Just like guided meditation, coloring allows us to switch off our brains from other thoughts and worries and focus only on the moment, helping to alleviate stress and anxiety. It can be particularly effective for people who aren't comfortable with more creatively expressive forms of art, like painting or writing. The participants who are more guarded find a lot of tranquility in coloring images. It feels safer for them and it creates containment around the coloring process.
The alternative is to buy coloring books in a shop or order the physical coloring book online. If you order the physical book online, it can take ages to receive it. Then you have to wait for the item to be delivered. If you buy a coloring product in digital form, you receive it within minutes of purchase. Normally, once your payment is made, an email is instantly sent to you and includes your purchase.
Coloring pages help a child to develop better fine motor skills as well as hand eye coordination. You might wonder if colouring inside the lines is important, yet you will soon realise that it is when you see your children use the same skills when playing sport. Creativity is your children's greatest achievements particularly when coloring online. Children gradually start to create their own images, style and color online. Children will often find numerous ways to be creative both on and offline. Experience and creativity is education and learning for everyone. If children are creative they will likely be open to new ideas and educational experiences.