Put a solid colored block on the top of your greyscale image and change the transparency mode of block from normal to "multiply".

in illustrator?

is that the only way

Illustrator, yes!

What exactly you want to do with your image? If you want it to be monotonous(tints and shades of one color), thats the only way in Illustrator. Since its not an image editing software, you cant play much with images directly.

There is another way, but it may not have quite the result you're looking for and involves Photoshop too: Basically convert your colour image to Grayscale (Image>Mode>Grayscale), then convert it to a Bitmap image (Image>Mode>Bitmap - set the output resolution at at least 300dpi). You then end up with a 1bit image: Take this into Illustrator and select the desired colour and drop it onto the image. You then have something akin to what you're looking for. The shame is that in Macromedia Freehand (RIP), you could drop a grayscale image (not a bitmap) into it and colourise it the same way much like Quark does/did, and it would be great. I'll never understand why Illustrator doesn't do this - especially now that Adobe bought up Macromedia!

Good luck

JH

If you are using Photoshop you can also add an adjustment layer (on the bottom of the layers pallet, click the black & white circle) then choose gradient map. Then choose white and your colour, you can then choose the amount of white verses colour by sliding the stoppers.

This technique is more useful for when you want to use more then two colours.

Also very useful for changing the colour of an object that is already full colour, say you have a red truck and you want to make it green. First use another adjustment layer to greyscale the section that you want to change (use a mask on the adjustment layer to isolate the section you want to change), then use the gradient map layer to have; black, your new colour say green, then white. Again use the mask to isolate the section you want, and try sliding the stoppers until you get a desirable outcome.

Sorry to digress.

Good luck

"The shame is that in Macromedia Freehand (RIP), you could drop a grayscale image (not a bitmap) into it and colourise it the same way much like Quark does/did"

^ Thats what I was talking about. Forgot the program name.

Seems like such a simple thing. ADOBE STEP UP YOUR GAME.

If you want a monotone colored image, you can create an opacity mask in illustrator.

Fill a rectangle with color behind the image, select both image and rectangle and choose Make Opacity Mask in Transparency palette menu.

To correct for the negative image, show thumbnails in same menu and check invert Mask.

Use the bars in the color palette for the color you want.

This is easy in CS3, but not in earlier versions. Open the file and select EVERYTHING visible. Click on the color guide icon on the right–it’s the one that looks like a fan of swatches. Within Color Guide, click the tiny color wheel icon (edit colors) to open the Live Color menu. Select the "Grayscale" color group and you’re done.