These are phrases that I know combined with some I found and learned about as well, I decided to add the meaning or suggestions with these in case you don't know what they mean. I kinda recommend making up small poetic phrases that fit the chapter and using the names of other shades to help title stuff. Like instead of the color red, use scarlet or crimson. Or instead of blue, use aquamarine and cobalt. Green, go with jade or viridian.. shades like those~
For black!
"Black and blue" referencing the colors of bruises (plus it's two colors for the price of one!)
"Black eye" a specific bruise that has a nickname (this one can be a metaphor as well)
"Jet black" meaning just.. black, like as dark as it can get
"Monochrome" a type of color scheme, but many people associate it with only black and white
For blue!
"Cordon bleu" meaning "blue ribbon" in French, high quality cooking (you can twist it to be a metaphor)
"Deep blue sea" ..this one's fairly obvious, but you can make it symbolic as well
"Blue blood" basically, aristocratic or royal blood
For grey!
"Silver lining" referring to the sun shining behind the clouds
"Grey clouds/ skies" referring to rain clouds or an overcast sky
"Silver bullet" meaning a direct and effortless solution to a problem
For red!
"Paint the town red" basically violent riots, referencing blood in the streets
"Red herring" meaning a finding led by misleading pieces of information
"Red in tooth and claw" referencing blood on predatory animals from hunting, or a violent world