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In section 107.1b of the Comprehensive rules there is an example where power and toughness of a creature change in a way that I find confusing.

Example: Chameleon Colossus is a 4/4 creature with the ability “{2}{G}{G}: Chameleon Colossus gets +X/+X until end of turn, where X is its power.” An effect gives it -6/-0, then its ability is activated. It remains a -2/4 creature. It doesn’t become -4/2.

I don't understand why the creature doesn't become a -4/2 creature. In my mind the creature has a base power and toughness of 4/4 which is first modified to -2/4 and subsequently to -4/2 (for a duration). It would seem to me that the only way for the power and toughness values to not change is if X is zero or the effect from the ability is ignored for some reason. But X is not zero since the base power has changed from 4 to -2. Neither is there a reason to ignore the effect from the ability.

Is the meaning of the text that a creature can "become" something else if it undergoes a change in characteristics which is separate from the creature being subject to modifier values that cause a temporary change in the interpretation of it's characteristics? In the example the base power and toughness of the creature is 4/4, which are modified by the first effect to -2/4, changing the characteristics of the creature? The Colossus thus "becomes a -2/4 creature". There is then the effect from the activation of the creature's ability which causes a temporary interpretation of the characteristics to -4/2 until end of turn. In this sense the creature "doesn't become -4/2" since the characteristics do not change from -2/4 but are merely subject to a temporary modifier value.

I'm a new player (haven't even played a game yet) trying to get an overview of the rules, so I might not be using the correct terms ("effects", "characteristics", "objects").

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The reason is rule 107.1b itself:

Most of the time, the Magic game uses only positive numbers and zero. You can’t choose a negative number, deal negative damage, gain negative life, and so on. However, it’s possible for a game value, such as a creature’s power, to be less than zero. If a calculation or comparison needs to use a negative value, it does so. If a calculation that would determine the result of an effect yields a negative number, zero is used instead, unless that effect doubles or sets to a specific value a player’s life total or the power and/or toughness of a creature or creature card.

Adding to power and toughness does not fall under any of the exceptions, thus 0 is used instead of −2.

(Under some earlier versions of the rules, the exceptions were different and did include this case of adding to power and/or toughness; it would have become −4/2 under those versions of the rules.)

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    OK, I think I see what is happening. "Calculation that would determine the result of an effect" here means "calculation of the amount by which the power will change", not "calculation of the eventual power". That's quite a confusing use of English - "result" of something normally refers to the situation after that thing has been applied. Commented Jul 15, 2022 at 12:06
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    @James The calculation is "add -2 to the creature's power". You can't add a negative number to power, so you add zero. The same way, you can't gain negative life, or draw negative cards. Commented Jul 15, 2022 at 14:17
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    @doppelgreener Where does the rule say you can't add a negative number to power? It explicitly says you can use a negative number in a calculation: "If a calculation needs to use a negative number, it does so". Commented Jul 15, 2022 at 14:32
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    The text of Chameleon Colossus's ability is "Chameleon Colossus gets +X/+X until end of turn, where X is its power." The calculation that would determine the result of the effect is calculating the value of X. If that calculation results in a negative number, 0 is used instead.
    – murgatroid99
    Commented Jul 15, 2022 at 16:05
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    Also, for context, they changed this rule a few years ago to prevent Death's Shadow from getting buffed beyond 13/13 when its controller has negative life. Death's Shadow has the ability "Death's Shadow gets -X/-X, where X is your life total." And again, the relevant calculation is the calculation of the value of X.
    – murgatroid99
    Commented Jul 15, 2022 at 16:09

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