I have stripped pieces out of that library so I haven't used it directly but one thought is that you are converting your colors on the fly. See what happens if you do:
unsigned int white = oled.rgb(255,255,255)
And replace that portion of your code with white. If that doesn't work, put a very slight delay in between the calls to drawstringblock. The Display takes a second to process the command you sent it and it might be missing that second one. Especially at 57600.
I had all of my standard colors call once and print to serial. Once I knew the values, I DEFINED them in the declarations section and stripped the code. The only one this wouldn't work for was Black which I call at setup for some reason, the string call does not like 0.