Since they (in line with most of the EEC) prohibited the use of red diesel fuel to power pleasure craft several years ago, there are concerns that the Belgian Excise Authorities are taking a keen interest in where and when you bought your fuel.
If you are planning to visit Belgium, ensure that you have detailed receipts for the purchase of any red diesel in your tanks, showing date, name & address & VAT registration of supplier, quantity, price and an explicit statement that it is supplied in the UK Duty Paid. Do not carry any red diesel in cans.
We already have one report of a UK yachtsman being fined 500 euros in Namur because he had tanks containing UK supplied Duty Paid red.
The fact that the UK authorities and the yachting industry reached the agreement to allow yachts to use red in 2009 does not seem to have been communicated to each and every excise officer in the rest of the EEC, who have been enforcing its prohibition for several years.
There is a useful explanatory document on the RYA website showing correspondence between the EEC and the RYA which sets out the legal position as seen by the RYA, with good translations into French & German, and well worth downloading and carrying with you.
Best of all, if you can fill up with white rather than red, do so.
If you do experience any difficulties on this issue whilst cruising in Belgium, please let the CA know at office @cruising.org.uk