I had a similar problem when I needed a compact small capacity back pack for use with my LowePro photo harness. So took myself down to the local surplus store and purchased a military style zip on/off side pocket made by Arktis which comes with the load bearing straps to make it into a 10 litre capacity day sack. Perfect and plenty big enough for what I wanted as it sits in the middle of the back and measures eight inches wide at it's widest but about eighteen inches high, £16 all in and it's made from decent material and waterproof, although there is a hole in the bottom for water drainage should the pack become immersed in water. A very well designed and constructed item complete with compression straps and attachment loops for whatever and at a very good price.
Being a zip on/zip off rucksack pouch, if I had another one, I could zip the two together to make a twenty litre capacity pack, but I am finding ten litres is plenty big enough for what I want.
For photo outings the pack is for carrying a water proof poncho and food, but the pack has now found another use, for it is perfect for cycling.
Colours, well it is military alternative gear, so colours were limited to olive green, black and cabbage, I avoided cabbage and managed to get black, which just happens to match my LowePro stuff, so well sorted.
But if anyone has any unusual requirements for gear, check out military alternative stuff, it is well made and very affordable as it is meant for the low paid squaddie to buy. Colours are limited to tactical, but green is ok as is black. Civilian market equipment always carries a price premium as it is classed as leisure instead of work.
