When I tried to write letters into color picker hex field it kept erroring. The letters were from a to f and we're right. What should I do?

Are you sure you didn't change your input language by accident to some invalid symbols (happens to me pretty often)? Are you able to input numbers only? What if you type out the code somewhere else like notepad and copy-paste it into the field? 

If nothing works, can you post a screenshot of where you're trying to input the value?