No, not necessarily.

If you used a latex paint, those yellow stains could very well be something called "Surfactant Exhudation", which is a fancy technical word for saying "the soaps in the paint are bleeding out of it".
Take a damp sponge and see if you can clean those yellow stains off. If so, it's soap you're cleaning up.

Surfactant Exhudation is caused by high humidity or rain occuring shortly after painting. The surfactants (pronounced "soaps") in latex paint will be attracted to the moisture on one side of the paint film and will bleed through the paint film to it's surface, causing yellowish or brownish discolouration of the paint.


The other possibility is that the wood you painted was cedar, redwood or Southern Yellow Pine. All of these woods have a lot of tannin in them, which gives them their distinctive colours. Tannins are soluble in water, and so if you primed with a latex primer, the tannins would have bled through the primer and you SHOULD have seen that bleed through on the primer before you painted. If you then painted with a latex paint, the tannins in the primer would have bled through the latex paint too.

If you can tell us what kind of primer you used (whether latex or alkyd), how long you waited after priming before painting, what kind of paint you used, and if there were any notable changes in the weather shortly after painting, then we can possibly help more.

But, it sounds like surfactant exhudation to me, and cleaning it off with water would confirm that. You should start to notice more and more bubbles in your cleaning water as you remove more and more soap from the painted wood decking. Surfactant exhudation causes no damage to the paint, and neither does removing the soap after the paint has dried.