'plain-colored' is a compound of 'plain' and 'colored'. 'plain' originates from Old French and ultimately from Latin 'planus' meaning 'flat, even', and 'colored' comes via Old French from Latin 'color' meaning 'hue, color'.

'plain' changed from Latin 'planus' → Old French 'plain'/'plein' → Middle English 'plein' and became modern English 'plain'; 'color' changed from Latin 'color' → Old French 'colour' → Middle English 'colour' and developed into the adjective 'colored' (US) / 'coloured' (UK). The compound 'plain-colored' is a straightforward modern English compound of these elements.

Initially 'plain' primarily meant 'flat' or 'level' (from Latin), but its sense shifted to include 'simple' or 'unadorned'; 'colored' has consistently meant 'having color'. Together the compound came to mean 'having a simple, unpatterned color', its current sense.