Thought I'd revisit this thread but for the Washington(ish) Services. Feel free to add your own ideas

56A: ???????
179/180: Little Pinks
183: ?????
185: ?????
186: ?????
187/188: CityRider
194: MetroLink
291/292: ????? 
293: ?????
294: MetroLink
393: ????? 
493: ?????
551: ?????
638: Country Ranger
777: CityLink
W2/W2A/W3: Little Pinks
W4/W5/W6: Little Pinks
X2: ?????
X4: ?????
X5/X15: Red Arrows
X6: Silver Arrows
X8: ?????
X85: ?????
X90: ?????
X94: ?????
X95: Fast Cats