Sunday, the ensemble number in the diner in Tick, Tick... Boom, is a direct homage to Sunday from Sunday in the Park with George. It is unsurprising perhaps that Jonathan Larson would pay homage to Stephen Sondheim, given that the latter acted as a mentor figure to him among many other writers - it's a relationship that features in Tick, Tick... Boom, which is on Netflix if you want to learn more.
But whilst a pastiche, it also signals something important about the style of musical that Larson was pioneering. I like to term this the automusical, in line with modern autofiction - shows that draw heavily on the personal experiences of the author, and which are often most concerned with painting a picture of the lives of their generation. The choice to pastiche a song describing a painting is an important one - there are many Sondheims he could have parodied, but you'll note that he picks one which is in its essence pictorial, a freeze frame, and Larson in Tick seems concerned with the idea of freeze framing a generation and feeling their feelings as though in stasis. Part of this may be wishful thinking, and the focus on capturing the moment before time runs out is likely less inspired by Larson's sense of a timer on his ambitions and more on the AIDS crisis. His wish to freeze frame a generation is borne out in Tick by the fact that we never see Jonny succeed or Michael, who is HIV+, die - the musical freezes them in limbo.
And thus, in this freeze frame, Larson also sets himself apart from Sondheim's concept musical, much more concerned with the question of art than anything else. The notes of Sunday are, after all, literally inverted (listen!) as though to pull away from the abstract into a deep rooted reality, pulling us down instead of up like Sondheim. Thus Larson separates his work from those who went before - pioneering the automusical.*
*term invented by me - let me know if there's a better one