If it's the yellow ones in the gallery on there you mean, then those boats are the Severn (FAB 3) & Trent (FAB 4) prototypes (17-01, O.N. 1179 & 14-01, O.N. 1180) in theiri trials period. As far as I can remember the R.N.L.I. were experimenting with different colour schemes to make the boats more visible as more and more non-lifeboats were being painted high vis orange (pilot boats etc.)
Change was not adopted in the end and the boats were painted the normal orange. After trials, the Severn became a training boat, and was sold off in 2005, while the Trent after a brief spell at Alderney entered the relief fleet in 1995.
Note the position of the flying bridge on both prototypes differ to normal production boats