The animators may have also found that using the Peter Parker outfit in the early days just made things easier in the planning process — having full intention of putting Miles in his normal black-and-reds when real material was being made.
Or perhaps there will be one sequence in the traditional colors, and most of the movie will have the Ultimate design.
That on the table, there are a number of explanations for what’s happening here.
The first is that the animation started to come together before a decision about the protagonist’s true identity had been made — meaning that the artists just started drawing the typical version of Spider-Man.