For Polishes:
I might say blue would be most neutral, since if you breed a blue + blue=splash, black, and blue. probably not a black unless you dont want any more khaki chicks since khaki and black make 100% chocolate chicks. Although ive never bred chickens, i just got some polish of show quality stock that i hope that i will breed and show them. below is a chart from my research of genetics from polishbreedersclub.com and possibly another site which i cant remember the URL. I've also included frizzling just in case you have frizzles. Sorry i dont have info for all the colors, just the basics:
Smooth with frizzle gene + Frizzle= 50% frizzle 50% smooth
Smooth w/frizzle gene + smooth w/frizzle gene= small percentage frizzles
Straight feathered + frizzle= Percentage frizzles, smooth feathered will have F-1 frizzle gene
Pick up 1/4th frizzle genetics each generation
Blue + Blue= 25% black(without gene), 50% blue(with 1 gene) and 25% splash(with 2 genes)
Black + Blue= 50% black, 50% blue
Black + Splash= 100% blue
Blue + Splash= 50% blue, 50% splash
Splash + Splash= 100% Splash
Khaki + Black= 100% Chocolate
Chocolate + Chocolate= Chocolate, Khaki, Black
Chocolate + Black= Chocolates and Blacks
Khaki + Khaki= 100% khaki
Chocolate + Khaki= 50% chocolate and khaki