Photoshop: converting to a one-colour image w/ transparent background

Question

I have the following image with a transparent background:

I would like to convert it to just a single colour, but for it to fade to transparent rather than to different tints of the same colour. A bit like the below — I understand this is an illustration and not a photo, but it should give you the idea:

I've tried a duotone / monotone image with blue, but that gives me blue and white... not blue and transparent:

I've also tried setting the image as a bitmap with diffusion dither. This gives me just one colour (as I'd like it) but it looks really noisey and poor quality:

Is there a better way of doing this?

Answer 1

• Convert your document to grayscale.
• Press Ctrl / Cmd + A to select all.
• Press Ctrl / Cmd + C to copy the image to clipboard.
• Convert your document back to RGB (or CMYK if you prefer).
• Make a solid color adjustment layer. Select the color you want for the lines.
• Delete or hide the original image.
• Alt + left click the layer mask of the solid color layer to enter "mask mode".
• Press Ctrl / Cmd + V to paste the image to the mask.
• Press Ctrl / Cmd + D to deselect all.
• Press Ctrl / Cmd + I to invert the mask.
• Click the solid color icon to exit "mask mode".
• Save your image as psd or tif with transparency.