Enamels are best for this and also best if mixed with some clear to make a translucent colour, that way you can layer it on and not block out too much light and get a richer colour.

Several light coats required... do your initial and let dry, then apply light source behind and it will show your flaws and then spray again.
If you use a light source at the start its nigh on impossible to not spray too much to try and obtain an even colour. First coat is best done blind

Thanks Gav - good info.
I am a little scared to do these as they are not re-available if I stuff em up 

Going off to buy the 2mm perspex for the backglass today.
I have worked out that there is enough clearance for that sheet before it would hit or be near the globes BUT I will have to cut the shapes out of it for all the displays etc as they sit fairly flush against the backglass normally.

Watch this space