Lovely work Charlie. 
I've tended to do my yew bows two different colours (oft exaggerated) to represent the heartwood and sapwood. It's been the case for me since the Perry's first produced their first Late Med. mini's for Perry Miniatures (as opposed to their "Poitiers" range for GW)- so, I think I'm saying I've been a button counter since then! 
Yes I considered doing the two colours as I've seen it done very well... but decided not to in the end.
It's interesting what things we consider important for our personal button counting! I don't feel the need for the bows to be a 100% accurate representation of yew longbows, but do think a lighter colour is much more appropriate than a darker one.
The exact colours we choose for clothing and uniforms... whether armour should be bright and shiny or dark and dull.... what colours we choose for bows and spear hafts... they're all personal choices that work together to produce an overall 'look' of our figures, whether they are bright and garish, or dull and muted, 'realistic' or not, etc....
Note all the leather (belts, gloves, pouches, etc) on my minis I paint the same light brown. I know this is probably unrealistic (there should be more variety, and more darker leather colours), but I don't feel the need to 'button count' it. I like the way the light leather colour 'pops' and makes the belts etc stand out against the often quite drab and muted colours I use for clothing and armour. I did start painting all the tall riding boots with turned down tops darker colours though...
But yeah, I'm very pleased with the colours I use overall... If you look and analyse too closely at any one element it can be accused of being 'unrealistic', but I think the overall effect is one of 'realism'. It's just the original darker colour of the bows had been bugging me for some time!