In oil painting, you want to keep things as simple as possible.
Remember there are only 3 rules to keep your painting in the best possible shape for a long time:
- Fat over Lean
- Thick over thin
- Slow drying over fast drying
An oil wash basically ignores the fat over lean rule.
I personally use a bit of White Spirit (similar to turps) on my first layer to wash and/or sketching the basic composition.
Painting wet in wet, I am forced to calculate the effects my ground colour will have on my eventual painting.
I also use a small mirror to check my composition and proportions throughout the first stages of a painting.
Advantage: all colors are bound together and there is more chance of unity in the overall composition.
Sometimes I use just a little bit of White Spirit on the 2nd layer -in any case less than on the first layer (= rule nr 2), but from the 3rd pass on I use paint only (rule nr 3).
If you want to start off with a toned ground but do not want it to mix with your 2nd layer, I strongly advise you to prepare your carrier with the desired tone at least a day up front so it is dry before you start. Use just enough paint with turps to make it dry faster.
Be careful with the phthalo colors, they are very strong pigments and are harder to control when painting wet in wet.