Flutter uses hexadecimal ARGB values for colors, which are formatted as const Color(0xAARRGGBB). That first pair of letters, the AA, represent the alpha channel. You must convert your decimal opacity values to a hexadecimal value. Here are the steps:
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Take your opacity as a decimal value and multiply it by 255. So, if you have a block that is 50% opaque the decimal value would be .5. For example: .5 x 255 = 127.5
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The fraction won't convert to hexadecimal, so you must round your number up or down to the nearest whole number. For example: 127.5 rounds up to 128; 55.25 rounds down to 55.
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Enter your decimal value in a decimal-to-hexadecimal converter, like http://www.binaryhexconverter.com/decimal-to-hex-converter, and convert your values.