I have a question about teaching reading. I often read in the forum that I should show my “flashcards” in categories. Now, my boy likes the animals and some of the foods, but could care less about colors and other things. 
I would like to slip those in once in a while. Is it bad to put a color flashcard in among the animals? Is it a problem if I have colors, food, animals and instruments all mixed up in the flashcards that I’m showing him (I usually have 3 groups of 5 cards that I show him a day) ?
I think it’s fine! Again, there are no hard and fast rules, and whatever works for your child!
Tip: You can just create a playlist to play different words from different categories.
I am sure its fine - there are so many groupings for words anyway - I could do a colours group and then a “words that begin with G” group and the word green could appear in both - one time with colours and another time with totally unrelated words like “grapes, gun and gerbil” for example.
As long as you are introducing the written word to your child, it really doesn’t matter whether things have been categorised correctly or not at all.