also i just saw this bit on this page
The filters are used to isolate a part of the visible light spectrum that is absorbed maximally by the sample. Different colourimeters use different sets of filters but typical wavelengths passed are red filter: 630-750nm, green filter: 510-570nm and blue filter: 360-480nm. Although you will normally be told which filter to use you should consider and understand the reason for this choice.
this is the page it came from
ok well hope it all goes fab - tell me if it does help then ill tell the world
In other words use the complementary colour to the compound you are measuring.
If you are told the wavelength it absorbs at, then use a filter that lets that wavelength of light through.
i used a blue filter and wavelengh was 470 nm