Printing the grey letters on clear decal sheet looks fine when printed, but once applied, they look like thin, very dark grey, almost black letters. You'd need to paint over them either before application, or once applied, fully set and cleaned.
If making decals, let the printed sheet cool and dry for around 30 minutes, then spray on a clear, gloss varnish (Do NOT use Johnson's 'Klear' for this!). Let this dry for at least 12 hours before using the decals as normal.
Note that all decals printed onto clear sheet from an ink-jet printer will not have the density associated with 'proper' decals, depending on colours, but can sometimes be improved by using two decals, one on top of the other, if this won't look too thick on the model. Decals printed from a laser printer, which requires a different clear decal sheet, can be slightly better, but still not up to commercial standards.
Before committing yourself, just re-check the size of the codes - 24 inch, 30 inch and 48 inch were the standard sizes, and Hurricanes normally had either 30 or 48 inch codes, with a couple of squadrons possibly using the smaller, 24 inch codes.
Sorry, I can't check my references at the moment - although my library is now back on the shelves, there's a bl**dy big cabinet in the way at the moment, so I can't get to the books!!