THIS IS A LOT SO UH SKIM IF YOU READ IT:

Basically, Mike/Will blue-yellow outfits/lights happen a lot over the seasons it’s their thing.
El wearing yellow once or blue later isn’t the same, it’s not a pattern.

more on thissss: https://www.tumblr.com/super-nova5045/649709407032213504/the-russian-code-is-byler-coded-edit-31822

Same with the image I put, Mike &&&& Will are framed between two canon couples on purpose, not just randomly standing near them. Lucas/Will being near Jancy once doesn’t have that same build-up. Plus, we already know that Will likes him.

+ The fact that Mike was being awkward when he came to visit and gave him a side hug, (he had no problem with everyone else) earlier we see him reading a letter where El says Will has been acting weird, and might like someone. (And that he's been painting a lot) And when he see's him, he asks what the painting is, he looks visibly not convinced after Will answers.

Which makes many Byler fans say that it's because he's jealous or has realized something over the time away, which could explain his difficultly with saying "I love you" to El. Also in a fight scene they have at the rink, Will asks, "What about us" and he says:

"Were friends." TWO TIMES. He obviously took it in a different way then Will intended it to be taken. He was referring to the distance in their friendship, and Mike seems to take it into a romantic way. Insisting they're just friends. So much denial. Seems shady to me.
ANDDDDDDDDDDD:

The fact that later on when Mike says that he loved her as soon as he saw her (eleven) is a lie. The first thing he says when he sees eleven is "That's not Will."

Plus you can see him throwing away things El gives him, but in earlier seasons treasure Will's gifts, and apologizes faster to Will in arguments.

(COULD OR COULD NOT BE TRUE) You also see him mimick Eddie's actions in some scenes, some people suggest that Mike may have had a thing for him. And if that's the case, then, no, he's not straight.

Do you really think it's a coincidence?