Well if thats the case then a tube of each color would be more than enough but I'm not sure the oil will stick to the unfinished plastic really well. Usually weathering with oil is done on an acyrillic base. You may want to paint the tank before using oils. As already stated above oil paints do take along time to dry also, sometimes weeks. After you use the oils you'll want to seal it with something like testor's dullcote or similiar. Otherwise your oils will smear if you touch it for quite sometime after you apply it. But if you are going to use oils you may also want to get some burnt sienna and give it an oil wash too. This will also help get rid of the toy like appearance and bring out the molded on details. You can use mineral spirits and mix in some burnt sienna with a touch of black til it looks something like coffee, it should be mostly mineral spirits and not alot of paint. Then apply it evenly over everything and then wipe it down. You know the mixture is good when you touch the brush to the surface and it flows along lines and crevices depositing pigment along the way. As long as you dont wipe to hard it shouldn't remove your acyrillic base coat. And you can go back with a q-tip dipped in mineral spirits to remove excess coloring on open flat spots to vary the coloration. Again don't press to hard or you'll remove the base coat. This takes some practice to get right and everybody has there own way of doing it that works best for them so you should try it on something else like a cheap plastic model if possible before attempting it on your tank. But once you get it down it will look good. Once your done you can give it a light coat of testors dullcote. This will washout some of the weathering so if it comes out lighter than desired you can always go back and do it again. Hope this helps