CP 2 = Nil Errors on Ishihara Test
CP3 = Errors on Ishihara Test but designated Colour Safe after passing approved Lantern Test
CP4 = Errors on Ishihara Test and designated Colour Unsafe after failing approved Lantern Test
If you pass any one of the approved Lantern Tests you will be designated CP3 and you will be given an unrestricted Class One Medical Certificate. The Lantern Tests are not easy to pass and the majority of people who fail Ishihara go on to fail the Lantern Tests. If you are a marginal failure (One or Two errors on Ishihara) it is unlikely you will pass the Lantern Tests - but not guaranteed.
Here's the RAF Eyesight standards which will explain a little better.
http://www.assoc-optometrists.org/up..._standards.pdf