See the Android Color resource documentation for reference.
Basically you have the option to set the transparency (opacity) and the color either directly in the layout or using resources references.
The hex value that you set it to is composed of 3 to 4 parts:
Alpha (opacity), i'll refer to that as aa
Red, i'll refer to it as rr
Green, i'll refer to it as gg
Blue, i'll refer to it as bb
Without an alpha (transparency) value:
android:background="#rrggbb"
or as resource:
<color name="my_color">#rrggbb</color>
With an alpha (transparency) value:
android:background="#aarrggbb"
or as resource:
<color name="my_color">#aarrggbb</color>
The alpha value for full transparency is 00 and the alpha value for no transparency is FF.
See full range of hex values below:
100% — FF
95% — F2
90% — E6
85% — D9
80% — CC
75% — BF
70% — B3
65% — A6
60% — 99
55% — 8C
50% — 80
45% — 73
40% — 66
35% — 59
30% — 4D
25% — 40
20% — 33
15% — 26
10% — 1A
5% — 0D
0% — 00
You can experiment with values in between those.
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