a photo would help a lot and it's not difficult to upload as long as you set the size at no more than 125 kb with e.g. Microsoft Office Picture Manager.
Having said that, a date of 1978 seems very much like Whitefriars. They made paperweights with 3 base colours: Dark Blue (DB), Light Blue (LB) and Green (G). Rarity of these base colours really depends on the pattern. There are some "lace spray" pattern weights where GREEN is the predominent base colour due to one particular example being made in large numbers. Generally though DB is the most common with LB second and G the rarest but these latter two can reverse depending on the pattern.
Values vary wildly based more on the rarity of the primary design. Within a desirable design, like the Large Butterfly, a DB example may be £650, an LB example £750 and a G example £800. These prices will also vary between private sales, auctions and retail sales.
I hope this helps,