I am a fan of beelinereader, I want to use same idea to implement a reader app,in their website it is written that their technology is patented as here https://patents.google.com/patent/US20150234788A1/en?oq=20150234788 Does using gradient in the text(possibly any other color combination that beelinereader doesnt show in their list),which also means using different color for each letter, violate their patent coverage ? It sounds little weird the idea that typing letters in different colors violates a patent.
It appears that the patent application was granted. You may want to read the claims carefully to see if the claims that you were worried about were allowed by the examiner.
Your use of different colors for each letter and the claimed use of color gradients for each letter may only be an obvious use.