These Colour charts can help identify gelcoat colours to the nearest RAL colour code.
Please note however that these colour charts are reference only and are typically used to choose colours for full GRP part manufacture rather than carrying out patch repairs.
Due to too many variables such as age, UV exposure, colour manufacturer and batch number differences in original colour it is almost impossible to complete an invisible perfect colour match in gelcoat when not using the same batch or supplier to that of the original.
However you can still use the chart to find the closest match possible and work from there to use other colours or to mix in and try to make your own colour shades or add white to lighten the shade as demonstrated in this video