A single shortcut that changes text color in Notes?
This isn't exactly what you were asking for, but perhaps it is close enough.

Just below the menu place where you bring up the color picker (Format->Font->Show Colors) are two other menu items: Copy Style and Paste Style. This is what worked for me:


Select some text in your note
Manually set the color, like you do now (via Show Colors)
Set the cursor anywhere inside the colored text (don't select any text, just put the cursor there)
Choose Format->Font->Copy Style
From now on (including across relaunches of the Notes app) you can select the text you want to color and simply use the keyboard shortcut for Paste Style!


Hope this works well enough for you.