so many things dealing with office work, documents and text have blue icons and a blueish artwork @default.
be it libreoffice writer, thunderbird, dolphin (granted, folders can easily be reocolored with color schemes), kate, skanpage, joplin,…
while all this helps with a coherent ui design experience, it’s not exactly fostering the ability to distinguish apps from one another by first glance.
a lot of pdf readers today feature red icons and artwork, we all know the reason. a red app icon suggests immediately “well this could have to do something with pdfs”. or lets look at it from the other perspective: a blue icon does not exactly tell anybody “hey i’m dealing with pdfs”.
so it might be worth considering to recolor the stylized part of the “O” in the okular icon from blue to red?