For "slap chop", which really means speed painting, I would recommend getting the following:

Two cans of spray primer - one dark grey, and one white. Prime the whole model with the dark grey, then spray just from above with the white. Spray the white from above and a little towards the face area, simulating where light would fall if you were shining a light above the model, erring towards the face area so the front of the model is lit. I would use dark grey spray primer instead of black. Black is very black and won't work well with your contrast paints. The reason you start with a zenithal prime like this is it creates the illusion of volume or shadow - their backs and shoulders will be lit up, while underneath their arms and their lower chest will be in shadow, because it won't get the white spray primer on it. This also saves you drybrushing a ton of models.

Check out mixing colors and a color wheel. You can make any color you can dream of with white, black, blue, red, and yellow. I would also get a neutral brown since brown is a hard color to make (although you can make it with all three primary colors). With these 6 colors you can make any color you might need just a little bit of without having to buy a whole separate pot of paint. I recommend vallejo brand paints because their good quality, cheaper than games workshop, and have dropper lids instead of games workshop's very bad lids.

Here is a good write up of the contrast paints: https://spikeybits.com/2023/12/gws-contrast-paints-how-theyll-really-look-on-models.html

There is literally a paint called "ork flesh." Probably start there. Maybe flesh tearer red for the shirts. Keep in mind that green and red and opposites on the color wheel, they may clash or look Christmasy. I would recommend neutral or more realistic colors for all of the cloth and metals, and use an accent color that you will use throughout your army on markings like face paint, or a symbol on their shoulders, or dags on their weapons. For example, I would paint these guys with black and brown clothing and gear, and use an electric blue accent color on their faces and weapons. Your plan of green skin, red shirts, and yellow pants is going to make them look very painful on the eyes, if that makes sense. But I prefer realistic painting. They are your models, paint them however you like.

The secret to metallics is to paint the whole thing a solid metal color in two good coats, then wash it with blacks and browns to get it looking grimey. I would buy a dark steel color, and nuln oil and/or agrax earthshade. nuln oil is blackish, earthshade is brownish, both are shading washes that you slather over things to shade them. I like vallejo gunmetal metal for this then washing with nuln oil for cleaner weapons, add a little earthshade for dirtier weapons. You could probably mix a dab of brown into nuln oil and get a good dirty wash without buying agrax earthshade.

Final bit of advice - wash your models gently with dish soap and warm water and an old toothbrush before you spray prime them. They will have all sorts of oils on them from the manufacturing process and your fingers, you want a clean surface for the paint to stick to. Let them air dry before spraying them.

-dark grey primer

-white primer

-basic paints (vallejo?): black, white, blue, red, yellow, maybe brown

-metallic paint - vallejo gunmetal?

-ork flesh contrast paint

-nuln oil shade or vallejo equivalent

-a standard brush and a detail brush