I had a bit difficulty understanding why weiß wasn't weißem (strong neuter dative) here and was weißen (weak/mixed neuter dative)... then I guess I realized, "im" would indicate a definite article, just wasn't really clear... and the English translation would have a definite article anyway. Still a bit confusing though. XD
yep. "In" can give the following noun the dative case (inside) or accusative (into). but because "im" is short for "in dem" it must be giving the dative case to the noun, and because we have one of the der/die/das/den/dem family of articles any following adjectives will end in -en. Same rule can be applied to am zum und zur.
It's in the Dative case.
[edited] See http://german.about.com/library/weekly/aa033098.htm , but note that the "ALWAYS" applies only when considering nouns with an article & an adjective. As duoderSie notes below, when there is no article, the adjective takes an ending based on its gender & number:
"Many German learners find the DATIVE (indirect object) case to be intimidating, but when it comes to adjective endings in the dative, it couldn't be more simple. The ending is ALWAYS -en! That's it! And this simple rule applies to adjectives used with either the definite or indefinte articles (and ein-words)."
This is not true. It is true about 95% of the time though.
When the noun has no pronoun then the adjective endings are -em (masc and neuter) and -r (fem). Example: "it is in boiling water" "es ist in kochendem Wasser" or "es liegt in kochendem Wasser"
In any case it is not the using the dative case that is "intimidating" but knowing when to use it: which preposition, is it a wo/wohin preposition, which verb etc.
in plural dative it is -en with or without article. Infact for plural dative most Nouns also take an N if it doesn#t already have one. Eg the green Tree = der Baum Nom Sing der grüne Baum Nom Plural Die grünen Bäume Dative Plural Den grünen Bäumen
without article Nom Sing grüner Baum Nom Plural grüne Bäume Dative Plural grünen Bäumen
Hope that helps