First apologies for the poor quality photos.
The first photo shows 2 25p stamps
The next shows the same stamps under UV short wave light. One stamp appears much darker than the other - any reason for this?
The next photo shows 2 different stamps under UV light. This stamp has variants with yellow and blue phosphor bands. The stamp on the left has blue phosphor band but the one on the right appears to have normal uncoloured phosphor bands. Any suggested reason for this?