I was just thinking about this too. For me having the Markdown use a different background colour would help with the users visual clues in understanding a document.
Having a class connected to the cell that related to the cell type Javascript, Markdown, HTML, TeX. Would make it easy for a notebook to have a styles css cell that could define that classes colour.
Having a way to adjust the colours would also help in showing which cells are more important or that a cell has a relation to another cell is not just boilerplate.
Might seem like a bit of a visual overload but my ADHD brain enjoys that kind of loading.
Or have missed something simple and overlooked that Markdown can use HTML and I could just use the desired colour in there.
<div style="background-color:#C0FDFB;">
It seems to work but very hackery as adding a closing div to the MD cell caused issues.
using a cell to create a colour makes it easier to change all the colour.
<div style="background-color:#${md_colour};">