100% agree, your first starting point should be some research, speak to your current users, find out what they struggle with, find out what they like, observe them using the app to see where they click and what they are trying to achieve.
If you already have a database of users then look at the demographics of your users and look at doing some market research based on these demographics, find out what people would want with an app. If you don’t feel confident doing this yourself then there are lots of companies out there who will do the market research/user testing and analytics side for you.
Also look at other apps which do the same / similar things. Analyse them.
Maybe do a heuristics evaluation on your current app, this will always flag up things which can, usaully, be fixed quite easily.
Also, don’t focus on colour and design yet. Once you have an idea of what the user really wants, you can then create user journeys:
‘As a user I want to do … so that I …’
From the user journeys you can then look at creating sketches, wireframes, prototypes, but test as you go - this is the most important thing!
Once you are happy with the wireframe/prototype and the testing has been successful you can then start padding it out with designs. Again if you aren’t confident here there are many graphic design agencies who will create designs based on wireframes and user journeys.
Hope this helps