Hi,
I always spray the waterline area first, this is known as reverse masking. I used Ford Frozen white which is not a full white as that would stand out too much at this scale. You can then mask off and spray the blue, I used Humbrol 15 for this, again, this is not quite so vibrant as the other colours mentioned but looks correct for the scale. Remask the waterline again and spray the red anti foul, I used red oxide from the halfords range. this does give a matt finish but can be buffed up a little. Tamiya tape was used for all masking. Be aware that the waterline is slightly wider at the bow of a Severn boat. If you study pictures of the forward draft marks compared to the stern, this is noticeable. At 1/12th scale, the waterline is 5/16th ins thick forward and 1/4ins thick at the transom. (OK I am a rivetcounter!! )
Hope all above helps. 17-09