Some colour combinations are complimentary which means, according to an ex-girlfriend, they have similar intensities. If you converted to greyscale they would be almost a homogenious blob. Like:

yellow and navy blue
cyan with red
=  or even:
cyan and orange (gawd!)
magenta in yellow

It will depend on the precise shades, your computer screen or in the case of print, the ambient lighting, and how glossy (AND that another thing!).
And of course we all see colours a bit differently. Not to mention the lexicon we have been exposed to. eg are European robins really red? The answer is yes, until the orange fruit became popular then we had a word for it.