Learn Russian: Художники создают картины цветными красками. - Artists create pictures with colored paints.

In Russian, the instrumental case is used to show what tool or instrument is used to perform an action. Because the artists are creating pictures by means of colored paints, we use the instrumental plural endings: -ыми for the adjective and -ами for the noun.

How do I know whether to use -ыми or -ими for plural adjectives in the instrumental case?

It depends on the last consonant before the ending. The adjective цветной has a hard stem ending in н, so it takes the hard ending -ыми to become цветными. If the stem ended in a soft consonant or a spelling rule letter (like г, к, х, ж, ш, щ, ч), you would use the soft ending -ими instead.

Is картины in the nominative or accusative case in this sentence?

It is in the accusative case, because "pictures" are the direct object being created. However, for inanimate nouns (objects, not people or animals), the plural accusative form looks exactly the same as the plural nominative form (картины). This is why the ending doesn't change here.

The infinitive for "to create" is создавать. Why is the -ва- missing in создают?