That will bleach out quickly in the San Antonio sun.
That is exactly what happened to this color historically.
This was pulled from a Spanish Ordnance manual by the Curator of the Alamo and sent to me.
"The Spanish painted their ordnance in a blue grey and not as some authors think grey. The pigment was cobalt blue and when exposed to the weather for years it has a tendency to fade to a grey. The cobalt blue was derived from cobalt oxide mined in southern Spain that was sintered with silica (sand) at 1200 C. It was a very stable colour and was popular too in the Spanish Naval Regulations from 1772. The metalwork was painted black. "