No; it's fine. Just a forgotten alternative; I've added it not.

weißes is the accusative neuter form of weiß. You need to decline the adjectives.

One way of looking at it is that because the possessive determiner mein does not conclusively indicate the gender it has to be indicated by the inflection of the adjective, in this case neuter. Check out the "mixed inflection" table in the following:

https://en.wikipedia.org/wiki/German_adjectives#Mixed_inflection

The possessive determiners mein, dein, sein, ihr, unser, euer inflect like ein and kein -- and so they have no ending in the nominative or accusative case for neuter singular nouns. (Nor for masculine singular nouns in the nominative, e.g. ein Junge, mein Saft.)

Just as we say ein Pferd and not eines Pferd, so we also say mein Pferd and not meines Pferd.