Both are Thai GL. Both are GL with focus on young people/high school students. We still don't know much about the plot for Shades and No Romeo since they are both original stories(not based on a GL novel.)
Both are Thai GL made by the same company and may have some of the same actors? Both seem to be quite edgy.
Both are Thai gl that take place in high school. Although Shades seems like very edgy series compared to 23.5 that is wholesome.