What terminal should I better use for the crimping of a cable with a cross-section of 1.5 mm²- red or blue?


Both variants suit you, as they both are designed for stranded wires with 1.5 mm² in cross-section.
Red-insulated terminals are designed for range from 0.5 to 1.5 mm², blue for 1.5-2.5 mm². It is understandable that it is easier to put the wire of 1.5 mm² inside a blue terminal, but red terminals are cheaper. So you are free to make your choice.