RAL is a colour space system that uses a series of numbers and letters to correspond with various collections of colours, using RAL before the number to denote a RAL colour. FS is a similar system where you use FS before the number corresponding with the colour. There are charts available that give approximations of the paint colours between the two systems, so you can use FS if RAL is not available.
Determine which FS paint colour you have so that you can find the closest match in RAL. Look on the paint can for the number and note it on a piece of paper.
Consult a colour conversion chart. For example, Paint Assistant gives various conversions between multiple-colour spacing systems, including FS and RAL.
Find your FS paint number in the chart and locate the matching RAL paint colour. Make a note of the number.
Keep records of colour conversions so you will have the information at hand when you purchase paint.
Some paint colour conversions are approximation. The colour will not always be the exact same one you want.