My favorite color combinations are either blue/orange or red/green. These are complimentary colors, meaning that they are opposite one another on the color wheel. The first thing you should know about color is that placing complimentary colors next to one another creates contrast. You don't have to use pure red and green. You can use lighter tones, such as pink next to dark shades of green. In my new painting I used pink and green for that dramatic effect.
After The Rain by Susie Tenzer buy this painting on Etsy
The second thing you should know about color is that warm colors advance and cool colors recede. I use this technique all the time. In the example below, the background looks black, but it's really a combination of black, Indigo blue, dark green and Tuscan red. I added several layers of Indigo blue, to help push it back. The green rose buds contain some crimson red, especially towards the front of the buds, to help bring them forward. You'll notice that the greens are different in each part of the bud. The inside of the buds are blue-green (pushing that part back) while the outer leaves are yellow-green.
The third thing you should know is that combining complimentary colors creates grey. I do this when making a shadow or toning down a value that is too strong. I love when shadows include many colors, not just black or blue. The shadow colors of the green rose buds contain dark pink and red plus a bit of dark green.
What are your favorite colors to use when you paint and why?