Hello, Is there a good explanation of how data types worked?
They don't work. At least OS3.x gfx datatypes don't work in 256 color mode due to a mind-bogglingly stupid design flaw. So if you are going to make a gfx datatype it can only work 100% in 24-bit mode, which should be ok for you as most ppl use 24-bit gfx these days.
What kind of datatype do you want to make btw?
