Thank you for sharing your question about paint. In a nutshell, polyurethane paint is a clear coating that can be placed on either painted or non-painted surfaces to give it a layer of protection against wear and tear. However, enamel paint is usually oil-based but now has water-based counterparts as well. It is hard-wearing paint and usually dries with a gloss finish.
Enamel paint can be tinted into many colours and is ideal for indoor use. I suggest bringing one of the doors to your store's paint specialist and get his advice on what would be the ideal paint for your kitchen door.
Looking at the picture of your kitchen it seems like it has a polyurethane finish on it. If it does here is a handy step-by-step guide that might help:
You should also find this step-by-step guide useful:
If you need more information please let us know, we look forward to seeing the start of your project.