Called Rayadillo, these were specially designed to be comfortable for soldiers in tropical environments.  Most versions allowed the butttons to be removed (they were held on by a ring similar to key rings) anyway the problem is painting the raydillo, which is usually white with blue  or black thin stripes that from a distance looks like a hazy blue.  the Spanish used many variations even within units, so that it can be hard to get a feel for correct uniform colors.  most people just paint a solid light blue...but to PROPERLY represent it with super thin lines would be the most correct.