Here is the problem I have.

A spell which is already in effect has some sort of resistance which means that you need a spell to counter the effect of another spell already in effect.. all fine.

The problem is what about the medium ? for example, you turn a rock in red for a duration of moon. Another magi PeTe the rock to turn it into dust. Will it have a resistance roll ? Does it resist automatically since the spell is not designed to dispel a spell or does it turn the rock to dust, ignoring the coloring spell ?

I would says it turns to red dust. but what if the spell was to turn it to a blue rock ?

I think that the rock turn blue if the penetration of the second spell was higher than the first. But this don't dispell the first, just "cover". so, if the blue rock spell finish before the red rock spell, the rock turn red.
If the penetration of blue rock spell wasn't higher, the rock stay red, but with the second spell waiting in it.
Imagine multiple layer...
This is different to use a magic to restore the target to the natural state. You simplu put 2 layer on a target, and the strong is visible.
Or, because the Art of Magic is impredictable, you could have a mixmax of the 2 spell, and a violet rock.

and sorry for my english

I think that penetration is the key. I haven't thought this fully out and I am thinking about the situation as I type. I also hope that more people join this thread, as the subject seems integral to understanding how penetration, magic resistance, etc., works.

I cast a spell that turns a rock red and then someone casts a spell that turns the rock to dust. Two different properties of the rock are effected. That I believe is a key element. You produce red dust.

I turn a rock red and then someone tries to turn it blue (sounds like a certamen duel).

If the penetration of the blue spell is lower that that of the red spell the spell fails. It does not go into a "waiting state". The spell either effects the material or it doesn't. In this case of lower penetration, the rock does not turn blue, it stays red.

Similarly, if the penetration of the blue spell is higher than the penetration of the red spell, the rock turns blue. The magic of the red spell is overcome by the magic of the blue spell.

If the penetrations are the same, the blue spell FAILS. The rock remains red. The spell either succeeeds or fails.

Here are some issues.

1) What if the red spell is level 10 and the blue spell is level 50? Is is still penetration that counts? Wind of Mundane Silence dispels on spell level + stress die total being equal to or greater than double the previously cast spell. Certainly a level 50 Wind... spell would dispel the 10th level (except on a botch).

2) What if the spell I cast effects multiple properties of the rock, e.g. turns it blue and to dust? I would believe that both need to succeed for the spell to succeed, although with spontaneous magic, you could argue for a partial effect.

3) I am also afraid of Mini-maxers casting low level spells on themselves (no thread hijack here to warping points), and arguing that other spells have to beat the penetration of that spell to effect them.

Thoughts, curses and prohibitions welcom.

"1) What if the red spell is level 10 and the blue spell is level 50? Is is still penetration that counts? Wind of Mundane Silence dispels on spell level + stress die total being equal to or greater than double the previously cast spell. Certainly a level 50 Wind... spell would dispel the 10th level (except on a botch)."

What is the right answer for this ? Do you need the penetration total of a wind of mundane silence to beat the casting total of the spell that is being dispelled ?

Just to add another taught .. what about Rituals. I feel that a ritual should be better at affecting another spell than a formulaic or spon spell & not just because of the Artes + philo (since you can also add those bonus to a formulaic spell)

Apparently spells in ArM5 do not have any Magic Resistance, and Penetration is no issue at all when affecting them.
Instead, spell interaction is determined by comparing the base effect level of a spell detecting, derailing, modifying or canceling another spell to that spell's level. The exact formula used depends on the spells interacting.

Compare for this e. g. ArM5 p.147, ReIm Gen 'Restore the Moved Image', or ArM p.157 CrVi Gen 'Shell of False Determinations', CrVi Gen 'Shell of Opaque Mysteries', InVi Gen 'The Invisible Eye Revealed' and many many others.

Unfortunately the guidelines for individual spells are not exhaustive, and sometimes the SG has to extrapolate. You can take this as a motivation to introduce house rules, even to introduce Magic Resistance and Penetration in spell interaction.

But the way following the spirit of the written rules most closely is to extrapolate from the given spell descriptions. So if a Magus IYC wishes to turn a stone blue that was turned red before for D: Sun, you can extrapolate from 'Restore the Moved Image' and require that base effect+10+stress die of the spell turning the stone blue must at least match the level of the spell turning the stone red.

Kind regards,

Berengar