Why does the colour of a varnish differ between how it looks before and after application as well as by-eye and through an iPhone? I opened a tin of woodstain varnish and was surprised to find that the varnish was a dark blue in colour not the mahogany which was stated on the tin.

The photograph does not do justice as to what I saw with my eyes.
The greyish/slightly mauve colour in the photograph was a darkish blue to the eye as shown by the added coloured circle.
Once used the colour (wood to right of tin) is as per the label on the tin.
Why the blue colour when viewed with the eye, is it a scattering by emulsion effect.
Why the change of colour when a photograph was taken with an iPhone?
 A: For water-based stains and paints, the color "in the can" does not match the color "on the board" after the stain dries. This is because the product is a water-and-resin emulsion that has a milky appearance when in the can, that disappears as the water evaporates and the resins cure.
As that emulsion disappears, the true color of the pigment mix becomes stronger than the emulsion effect and eventually the color on the board is due just to the pigments and the board matches the color sample on the outside of the can.
Paint shops now use a spectrophotometer linked to a computer which is linked to the electric pigment pumps in the shop's prep area, so a customer can come in with a board painted with the desired color and the employee can then scan the board and derive a perfect match to it with a fresh batch of paint. The software that controls the pigment dispense takes into account the color shifts that occur during the drying process- so the freshly-opened paint can does not match the board, but it does after the paint is fully-dried.
