The 1955 season pre-dates the establishment dates of both franchises, the clubs will be wearing the uniforms of the local Pacific Coast League teams from that year.
The Angels will be wearing the uniforms of their PCL namesake, minus the “of Anaheim” part. It features a blue cap with interlocking white and red “LA” on the front. The jersey reads “LOS ANGELES” arched across the front in red.
Seattle will be wearing the uniforms of the Seattle Rainiers, the jersey has ‘Rainiers’ across the front in red lettering and a single red stripe on either sleeve. The cap is red with a white ‘R’.
Special thanks to @Wild_Halo on Twitter for the heads up on the Angels pic