Here's an example knarr (wafflecat's, from the
Building Color/Design Guide thread). Cypress for the blocks, planewood for the rest. Should give you an idea of what to expect when building your knarr:

This excerpt from the (extremely incomplete)
Variable Materials page on the wiki is a good general rule of thumb:
Note that you need a significant presence of a second wood type to get a result. 59 blocks of poplar and 1 of cherry will, for example, mean that poplar is used for both materials set by the woodblocks. A 35-25 ratio (Not entirely sure where the cutoff is) will mean that one of the materials is set by poplar, and the other by cherry.
To ensure proper materials display, it's wise to use a 60-40 split for primary/secondary materials and a 50/30/20 split for primary/secondary/tertiary materials.
edit: Also, good luck! Knarrs are fun to sail in. If you haven't fucked with dyes and custom image sketches yet I highly encourage it. The pigments aren't too hard to make and having your own sigil feels good. Here's my knarr using chestnut + planewood, just built it this past Thursday:

(sharper eyes may notice that's not technically "my" sigil but the sigil of House Clegane)