Why would a yellow rose bush produce red roses?

I've had 2 yellow climbing roses for 4 years. This year when they bloomed they were red! (One was multicolored pink). We had a severe drought last year. Did that effect it?

Answers:
1. main plant died and it cane back from its root stock
or
2. you need to check your soils Ph and adjust, roses like a very slightly acid soil best

Other answers:
it is confused? or maybe the birds and the bees cross pollinated the roses.
it is confused? or maybe the birds and the bees cross pollinated the roses.
The cross pollination could happen if someone in your neighborhood planted some other color of roses. Most likely it is something you added to the soil. Or it could be something that four years of growth depleted from the soil that the plant needs to to produce the yellow color. Roses are very fickle.
the severe drought killed the yellow rose graft on top, but somehow the rootstalk survived.
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