Are all (pure) alkanes, alkenes and alkynes - tasteless, odourless and colourless?
Why would a mixture of two odourless alkanes have an odour?

As already pointed out by Greg E and Uncle Al, numerous hydrocarbons without any (heteroatom-based) functional groups do show characteristic smells.

Naphthalene is always remembered for the typical smell of grandma's mothballs.

More pleasant smells are found in the group of monoterpenes, i.e. in those molecules with the sum formula $\ce{C10H16}$ that are derived from the combination of two isoprene units. 



These volatile compounds were/are used as fragrances for perfumes. Typical examples are myrcene (1), $\alpha$-terpinene (2), limonene (3), $\alpha$-pinene (4), $\beta$-pinene (5), $\alpha$-thujene (6), sabinene (7), and camphene (8).



As far as the colour of hydrocarbons is concerned:



A sufficiently conjugated system is far from being colourless, take lycopene (9), the deep red coloured dye in tomatoes as an example.