For 3d printed resins, you will likely want to go with acrylic paints. As @Spotty_Boots notes, you will want to prime them first, but the absolute first step will be to wash them thoroughly with soap and water. I use a soft tooth brush to get down into the crevices without causing damage to the minis.
Next you are going to need to sand them down, and that is going to be the most tedious part of the process. I use a combination of a hobby knife, metal files, and fine sanding sticks. The amount of sanding it takes to get 3d prints looking smooth is the primary reason I tend to avoid them unless absolutely necessary. Once you are happy with the sanding, wash them again to remove dust, and give them plenty of time to dry.
I mostly use Vallejo paints as well, they are pretty high quality, and they have a variety of lines, some are for airbrushing, while others are designed to apply with a brush. I mostly use the Game Color line, they are formulated for miniatures, and made to apply with a brush. That said, the Vallejo Surface Primer that @Spotty_Boots mentions is made for airbrushes, but works great with a normal brush. I know a lot of hobbyists who swear by it. I tend to agree, and have hardly ever touched an airbrush. You will likely need 2 coats if you apply by hand, however.
The problem you are going to have is with color matching the metallic paints used on the bots. Most paint lines have color charts available to help you match the colors you need, but even if you know the exact color of paint they used on the bots originally, metallic paints can be difficult to judge with a color chart.
My sense is that Crow’s gold paint is probably going to be easy to match, but Tom’s red metallic paint is going to be more difficult. Also, as far as I know, Vallejo does not have a red metallic in the Game Color line. They have a Mecha Color line that will likely have what you need, but it is formulated for airbrushes, and I have little experience with them. I have only used their weathering paints in that line for rust effects. Seems pretty decent though.
The Army Painter has a decent variety of metallics in their Warpainter line, but I have had mixed results with them, and do not recommend The Army Painter. Citadel paints from Games Workshop are pretty much the gold standard, and used to be my go to. They have a huge variety of colors, so you should be able to find what you need, but are expensive, and even though their bottle design is unique and has a neat dipper lid, they do not hold up well, and do not seal well. I had way to many problems with the lids breaking, and/or the paints drying up before their time.
You are probably going to need 2-3 coats of paint for a good look, and your first coat should not be in the final color you want the mini to be. Your base coat should be a darker, non-metallic version of the color you are going for. You can also base coat with black paint, but may need to layer more color on top of it if you don’t want things turning out too dark.
It sounds like you are going to have a number of models to work with, so that is great, you can experiment, and see what looks best. Even if you only had one model, if you don’t like the look, you can strip it down with isopropyl alcohol and start over. This will be a lot easier if you haven’t sealed the mini yet.
One piece of advice at this point is that with minis, the perfect is definitely the enemy of the good. Unless you are a Gold Demon award winner, your mini will have flaws in the painting. You will need to accept that, and at some point decide that your mini looks good enough. Remember, a mini should look good when it is viewed from across a table, not when it is right up in your face.
Once you are happy, you will need to seal your mini to protect the paint job. I usually wait until I have a dozen or so minis done, and seal them all at once. I use Vallejo Matt Acrylic Varnish, but it can dull some paints, especially metallics, and I think you would want a satin varnish for your project, possibly even a gloss varnish for Tom. You might even want matt varnish on his skirting, and something glossier on his chassis. That Mecha line I mentioned has a varnish that should work well with the metallics in that line, but I have no experience with it. Regardless, I would do 2 coats of varnish, with 12 hours to dry between coats.
Happy painting!
