Can I identify the colour of objects 5mm in diamater with an EV3 Colour sensor?
You may try to use the color sensor in RGB mode using a 3rd party block found here: http://mindcuber.com/mindcub3r/mindcub3r.html#ColorSensorRGBBlock
With this block you get a set of three 0-255 values, one for each R G B color. It will still be tricky to correctly classify the color of your objects as the readings are noisy - you can try to set ranges to each RGB value with or w/o normalization - but at least you have some data to work with.