Once printed, areas of colour in your artwork which have vector fills will appear differently to bitmap fills, even when the colour values are identical. This creates a visible join between the elements where vector colour meets bitmap colour, and will become more pronounced if RGB colour has been used as RGB will be automatically converted to CMYK. Instead, extend your bitmap colour out in a bitmap package, such as Photoshop, to avoid the joining of vector and bitmap colours completely.
Learn more about RGB Black and CMYK Black.